De Munitionibus Castrorum (“Concerning the fortifications of a military camp”) is a work by an unknown author. Due to this work formerly being attributed to Hyginus Gromaticus, its author is often called “Pseudo-Hyginus”.
Pseudo-Hyginus wrote sometime in the last half of the second century, between the reigns of Trajan and Septimius Severus. He says that what he has prepared is an explanation of his new way of laying out a camp which, he hopes the emperor will agree, is an improvement upon the customary way. In modern terms, it is a bid, or a proposal, or a sales pitch. Put together by an engineer. Due to possible errors in the copying of the original, or even additions to it by unknown hands, it is difficult to know just exactly what the original was, but the version that has been passed down is not well organized, is incomplete, and is not even internally consistent.
The goal of “understanding” the camp he describes is almost too ambitious. There are so many errors and omissions that is is difficult to recreate the camp he envisioned with any degree of certitude. Nevertheless, it is possible to extract some general information. It is worth the effort, however, since Hyginus is the only source we have for the Roman camp after Polybius, who described the camp of manipular army at the time of the Scipios, and a few brief paragraphs in Josephus.
Three Latin versions of De Munitionibus Castrorum are available online at Forum Romanum, IntraText and The Latin Library. This translation below is based on the 1977 Teuber text of Grillone and the 1979 Bude text of Lenoir.
The Main Areas of the Hyginian Camp
The general layout of the camp will be given first so that the various elements can be considered within a larger framework. However, many of the aspects of this layout are questionable and will be discussed in later sections when the necessary background material is presented. The illustration below shows the general camp layout with its significant parts.
Rampart and Gates
The Fortifications of the camp consisted of a rampart, a protective wall of turf and stakes and a ditch on the outside of the wall. The camp had 4 gates. If there were more than 5 legions in the camp Hyginus says there were two additional gates added at the ends of the Via Quintana. The Porta Praetoria faced the enemy. Ideally the ground sloped from the Porta Decumana down to the Porta Praetoria.
Readers who know Latin will have noticed that the right (dextra) and left (sinistra) gates are reversed. This is because Hyginus visualized the camp with the Porta Praetoria at the bottom whereas modern representations always put it at the top. In a similar vein above (supra ) and below (infra ) are also reversed in our frame of reference.
An open area 60 pedes wide between the Strigified Area and the Rampart. 
The area within the camp in which the tents were pitched. The term comes from striga which was a row 60 feet wide within which two lines of tents were pitched. The hemistriga, literally “half-row,” but better understood as one side of the row, was 30 feet wide and held one row of tents. It is the term used more often by Hyginus.
A narrow street 30 pedes wide which encircled the camp between the outer ring of cohorts and the inner tenting areas. 
Main Streets: Via Praetoria, Via Principalis and Via Quintana
The Via Praetoria, Via Principalis and Via Quintana were broad avenues providing access to the main gates. The Via Praetoria and the Via Principalis were 60 pedes wide.  The Via Quintana was normally 40 pedes wide but was increased to 50 if the camp were large enough to require additional gates at either end of the street.
There are lesser streets referred to. Hyginus sometimes calls them viae vicinariae, other times he does not name them at all. Some were 20 pedes wide, some were only 10.
This was a central area, 720 pedes by between 160 and 220 pedes.  In it were to be found the altars, the enshrinement of the army standards, the auguratorium, the tribunal. 
During the Republic the Quaestor was a magistrate who accompanied the army. Under the empire the office disappeared but the name persisted. The space was used for prisoners, hostages and plunder. The Quaestorium was narrower than the Praetorium. 
Three Parts of Camp
Hyginus identifies three main parts to the camp. The Praetentura was the area between the Via Principalis and the Porta Praetoria. The Latera Praetoria were the areas on either side of the Praetorium. And the Retentura was the area below the Via Quintana. Hyginus places the valetudinaria (hospital), veterinarium (veterinary hospital) and fabrica (workshops) in the Praetentura.  The length of the Praetorium appears to be fixed at 720 pedes. That would imply that changes in the length of the camp would occur in the Praetentura and/or the Retentura.
Translation of De Munitionibus Castrorum
Pitching of Tents
1. We will now explain the way in which the cohorts described above pitch their tents. One tent occupies ten feet; this length is increased by two feet for the pitching, and it shelters eight men. A complete century has 80 soldiers, so there will be ten tents which will run in a line 120 feet long. Because the width of the hemistriga is 30 feet, 10 feet are assigned to the tents, 5 feet to the weapons and 9 feet to the pack animals. This makes 24 feet; twice this is 48 feet. So when two centuries camp opposite each other, a striga (strip) of 60 feet will be made; there remains 12 feet which will provide sufficient space for those coming and going. This space is calculated for a complete legionary century. Since 16 men from each century are on guard duty at any one time, they do not pitch more then eight tents per century. In this way their centurion has a place to pitch his tent on the same area as those tents would have been. Otherwise it would have been necessary to allocate more space.
2. Because they are the most trustworthy of the provincial units, the legions should camp next to the vallum, to guard it and by their number to hold inside a human wall the army raised from foreign tribes. However when there are more reinforcements (non-legionary troops), when it is necessary to increase the allocation to the cohort, the width of the hemistriga is retained, the area remains unchanged, and we will alter the following: the area which was 120 x 180 feet will be 90 x 240 feet according to the diagram below, or 60 x 360 feet, as is also shown on the diagram. For one cohort occupies a space of 30 x 720 feet. Now, whenever the width is doubled, the length is lessened by the same proportion. But if we receive more legions and fewer reinforcements, as it will be necessary to camp the cohorts more tightly round the ramparts, we will alter the site thus: that which is assigned to the standards will be at the far end, and we will not change the method of pitching tents in hemistrigae. We have attached a plan of the arrangement below. Sometimes it is customary to assign an area 150 x 150 feet to a cohort, but although this can be done, it should be avoided, because the centuries cannot pitch their tents in the usual order and in one corner the area of the cohort’s allocation will be left uncovered, as indicated below on the diagram.
The Placement of the First Cohort
3. The first cohort camps inside the via sagularis, because of the standards and eagles and, as it is of double strength, it will have a double assignment of space; for example, 120 feet for the lines, 360 feet for the camping space, or 180 feet for the lines and 240 feet for camping; the arrangement of the plan will be the same for the other cohorts. Therefore if there is an uneven number of legions to be camped, in this case three, two of the first cohorts should pitch camp on the sides of the praetorium in a line next to the via sagularis, and the third in the praetentura, also on the sides of the via sagularis, on the left hand side as one is entering through the porta praetoria; one cohort camps on the right, opposite it so that the army can be led out in its usual order.
Five or Six Legions
4. However, when there are five or six legions, two first cohorts should camp on either side of the praetorium and two in the praetentura above the hospital tents, then vexillations (or a second cohort). If the situation calls for it, a quingenary infantry cohort may be placed instead of the vexillations, and if the space is even more restricted, it should be assigned to a legionary cohort, but this should be calculated exactly, so that 120 feet are assigned to the hospital and other departments which camp above (the first cohort), namely the veterinary hospital and the workshops, which ideally should be placed at a distance from the hospital so that there may be peace for those convalescing in the hospital. The area assigned to each of these departments is usually calculated as that for 200 men.
5. Legionary vexillations should be assigned the same space as a legionary cohort, which is calculated at 600 men, because of their baggage. They should camp above the praetentura or on the sides of the praetor-ium as stated above, above the first cohorts. If possible, they ought not to camp near to the rampart, because their legate would not be of equal rank and if the rampart happened to be overrun by the enemy, the legion and its legate will allege that the vexillations there are responsible.
Siting of the Praetorian Cohorts
6. The Praetorian cohorts should camp on the sides of the praetorium and should be assigned a double space because they use larger tents. The primipilares and re-enlisted veterans should also be assigned a place in the same area.
Siting of the Praetorian Cavalry
7. The Praetorian cavalry should camp on the right side of the praetorium, the emperor’s equites, singulares on the left. If there is a greater number of the latter, for example 600 equites singulares and 300 Praetorians, 150 of the equites singulares should camp in the lines of the Praetorians. It should be so arranged that there is an equal number stationed on each side and the decurions and other principales who have two horses each, will have more room to camp. If the number is less and there is enough room in the hemistriclae for 100 cavalry there should be no hesitation in assigning the area to the headquarters staff who are nearest there on the left side.
8. If there should be an uneven number of Praetorian cohorts, Praetorian cavalry are camped in place of one of the cohorts, since the number and arrangement of the cohorts on the right and left sides of the praetorium should be the same. If the equites singulares should number 800 or 900 they should camp in equal numbers and in complete lines on either side of the praetorium; if they number about 500 one line will be sufficient for them.
Length of the Praetorium
9. We must be particularly careful that not more than 720 feet is assigned to the length of the praetorium. Thus the Praetorian cohorts and the other unit which camps on the side of the praetorium will be assigned complete lines in a very satisfactory way. For although 140 to 220 feet may be assigned to the total width of the praetorium, 720 feet must be assigned to the length, as stated above. 20 feet of the length of the praetorium should be given to the guard post but if necessary ten feet is enough.
Siting of the Praetorian Prefect
10. In the same way an area from 50 to 70 feet can be assigned to the companions of our emperor, and in this area a space should be assigned to the Praetorian Prefect immediately adjacent to the via Principalis. Then when a road has been inserted, the Praetorian cohorts and the rest of the units should be assigned space proportionately as we have indicated in our pamphlet.
Siting of the Altars and Auguratorium
11. The altars are set up at the end of the praetorium; we will assign the auguratorium [where auguries were observed] to the right side of the praetorium next to the via Principalis, so that the general can observe the omens there correctly; the tribunal is set up on the left side, so that having observed the omens, the general can ascend the tribunal and address the army on the favourable auspices.
12. At the entrance to the praetorium and in the middle next to the via Principalis is the spot named groma, called this because the troop assembles here or because when the measurements are being fixed the iron-footed groma is put over the same place, so that by sighting from this point the gates of the camp form a star. For the reason written above, those who practice this technique are known as gromatici.
13. Access roads are provided running parallel to the via Sagularis so that the army can advance quickly for a sortie.
14. I will now describe the arrangement of the praetentura. The via Principalis, which runs between the right and left gates, and which gets its name from the principia, should be 60 feet wide, the same width as the work which separates the vallum from the legions and for this reason it is known as the intervallum. Likewise the road which leads to the porta praetoria (the via praetoria is no doubt named after the praetorium) should be 60 feet wide, as stated above, and because of the orientation of the tents, the lines situated above this in the praetentura should not run parallel to it, since the standards should look down the via praetoria.
Siting of the Legates & Tribunes
15. And so below the via Principalis we will assign the legates their space, which is termed scamnum and does not have the fixed measurements of the common lines because the number of legions is variable; but a space of 50 to 80 feet wide should always be kept for it, according to the number of legions present. The tribunes of the Praetorian cohorts usually camp in this area. In the same way an area below this should be assigned to the legionary tribunes, which is similarly termed scamnum. Separated from these by a road are the milliary or quingenary alae one after the other; we have shown below on the plan how each unit should camp.
Description of the Milliary Ala
16. As I have come to a suitable point, I will now describe the milliary ala. It consists of 24 turmae, in which there are decurions, duplicarii and sesquiplicarii, one of each in every turma. The decurions each have 3 horses, the duplicarii and sesquiplicarii 2. Thus there are 96 horses over the 1000, which is the number calculated when the extra horses are discounted.
Description of the Quingenary Ala
A quingenary ala has 16 turmae, decurions and other NCOs and so there are 64 additional horses. For this reason 3 feet are calculated for each trooper; their number is established at 1000 so that the prefect of the ala can be assigned an area in the space and their principales can camp in a more spacious way; for the rest, each trooper is assigned 2.5 feet.
17. This, however, concerns the retentura: the road above the praetorium, through the extension of which, when the army is larger, ie: 5 or more legions, the portae quintanae are usually placed, should be made 40 feet wide; if the gates are placed there it should be 50 feet wide, and it is known as the quintana because of the forces.
Siting of the Quaestors
18. The quaestorium is so called because the quaestors were sometimes assigned their places there; this is above the praetorium in a line with the gate called decumana because the 10th cohorts camp there. The quaestorium should be narrower than the praetorium so that the lines of the guard will be immediately behind the praetorium. In particular, the enemy’s ambassadors and hostages should camp there, and if any booty has been captured, it is placed in the quaestorium.
Siting of the Centurions of the Guard
19. The centurions of the guard should camp on the sides of the quaestorium close to the via quintana so that the rear of the praetorium is protected and they are right next to the praetorium; we will assign a double space to them because they use the same tents as the Praetorian cohorts. A quingenary infantry or part-mounted cohort should be placed above these, because of the size of the line. Infantry or part-mounted cohorts should camp in the other lines, looking towards the via quintana; the allies and other allied tribes should camp above these; it should be arranged in such a way that the allied tribes are held in all sides, as stated above.
20. 30 feet will be wide enough for the via sagularis; if there are five legions, however, it ought to be 40 feet wide. The offices of the first cohort, where the orders are given to the legion, should be placed in the scamnum of the legates opposite the eagle.
Proportions of the Camp
21. As far as possible, the camp should be 3×2 in proportion so that a blowing breeze can refresh the army. I said above a 3×2 ration, for example, 2400 feet long by 1600 feet wide. If it is longer the trumpet call can be sounded, but in a disturbance the horn cannot be easily heard at the porta decumana; if it is wider, the outline is too near being a square.
22. I think we have dealt carefully enough with everything that is necessary, and if explanations on some matters become necessary, I will deal with them in their place. In addition, lest I seem to be passing over the fortifications of camps, the choice of site, the fixing of measurements and the art of avoiding unfavourable sites, I will deal with them briefly at the end.
23. Meanwhile, I will explain the beginning of measuring and we shall review the units to be placed in the plan. We will also advise on where they should camp. The Praetorian cohorts and cavalry, the emperor’s equites singulares and the milliary or quingenary alae should camp on the sides of the praetorium if the space permits; then vexillations and second cohorts or quingenary infantry units above the first cohorts.
Troops of the Praetentura
24. Milliary or quingenary alae, Moorish cavalry and Pannonian light horsemen camp in the praetentura. All the marines camp there as well because they are the first to go out in order to construct roads, and so that they are safer whilst working, they are protected by the Moorish cavalry and Pannonian light horsemen; they should camp next to the cohorts. In the same way the scouts should camp in the line of the first cohort just like the legionary vexillations.
Troops of the Retentura
25. The milliary or quingenary part-mounted units camp in the retentura; I have explained their organization below. Each provincial (auxiliary? ) soldier is assigned one foot plus a fifth of the total length of the hernistriga; each cavalryman however receives 2.5 feet and the fifth. Now when we have received the units, in order to calculate the area of the retentura, we transform, as it were, the part-mounted units into infantry units so that we can assign the space to the cavalry with their cohorts more easily.
26. So a milliary part-mounted cohort has 240 cavalry which I shall transform into infantry, and to which I shall assign the 1 foot the infantryman gets and the 2.5 feet which is assigned to the horse. Then, taking half the number, it should be multiplied by five. We will deal with the number of cavalry in this way; this makes 120, multiplied by 5, this is 600. Therefore minus the cavalry, 760 feet is assigned to the remaining infantry of the milliary part-mounted cohort; added to the above number, the total is 1360. So let us remember that a space for 1360 men should be assigned when calculating the space for a milliary part-mounted cohort.
27. A quingenary part-mounted cohort contains the same organization but half the numbers of a milliary part-mounted cohort. So a milliary part-mounted cohort has ten centuries of infantry, 240 cavalry, (? ) turmae and ?6 decurions; they camp in 136 tents and from this number, the centurions and decurions use one each. A quingenary part-mounted cohort has 6 centuries, and half the rest of a milliary unit.
28. A milliary infantry unit has ten centuries and camps in 100 tents, of which the centurions use one each. In the same way a quingenary infantry cohort has six centuries and the rest of its organisation is as above.
29. We will place the tribes, Cantabrians, Getuh, Palmyrenes, Dacians, Britons, the centuries of guards and any others among the allied forces, in the retentura. We will assign five feet to each of the camels with their drivers. If they are to be used against the enemy they should camp in the praetentura next to the marines, but if they are there to carry booty, they should camp in the quaestorium.
30. And so we may calculate the numbers of the units which I have listed above: 3 legions, 1600 vexillarii, 4 Praetorian cohorts, 400 Praetorian cavalry, 450 of the emperor’s equites singulares, 4 milliary alae and 5 quingenary; 600 Moorish cavalry, 800 Pannonian light horsemen, 500 marines from the fleet at Misenum and 800 from Ravenna; 200 scouts, 2 milliary part-mounted cohorts and 4 quingenary; 3 milliary infantry cohorts and 3 quingenary; 500 Palmyrenes, 900 Getuli, 700 Dacians, 500 Britons, 700 Cantabrians and two centuries of guards.
31. After receiving the numbers we should always do the calculations for the retentura. so that we know how many hemistrigae to put up in each half of the retentura. Now, the number that will camp in the retentura is 13640. The number is halved, so that an equal number camps on each side, that is 6820. Now we shall organize the sides of the praetorium and calculate likewise for the praetentura just as we did for the retentura so that we know where we should assign space for the tents and standards of the legionary cohorts.
32. So we should notice that when 3 legions with their reinforcements are to camp, the half part of the camp is 720 feet wide and we assign 90 feet for the tents and 240 feet for the standards on the sides of the camp to the cohorts so that, having deducted the space for the cohorts and the width of the via sagularis, 600 feet remain. Thus milliary alae should camp in an area of the praetentura. Now to establish the assignments for the rest of the 600 feet space, we shall fill one side of the praetorium so that we know how many alae may camp in the praetentura.
33. 420 feet on the side of the praetorium is occupied by units of soldiers, 60 feet by the Praetorians, 20 by the guard picket, 60 feet by the emperor’s companions and 40 feet by the roads; this is approximately the distribution on this side of the praetorium; that is 600 feet.
34. Now to organize the praetentura we must compute the number of cavalry from alae which are left. This is 4000, half of which is 2000. A milliary ala should be assigned 150 feet for the standards and 600 feet for the tents. By this method 150 feet makes 5 hemistrigas. A cavalryman is assigned 3 feet in a length of 600 feet; I will take a third of this so that I have the number of cavalry who will camp in the length; that is 200 so this will be one hemistrig. And now we calculate 5 heniistrigae; 5 times 200 is 1000, the correct space for a milliary ala.
35. However, we must calculate the remainder of the force, as on the retentura, so that we know similarly how many hemistrigae should be set up. This number together with space for the hospital, veterinary hospital and workshops, which are calculated together at 600 men, is 8000. Half of this is 4000; this will be the number of half the part; it is 600 feet wide, which will hold 500 men; as we have said, an infantryman receives I foot and a fifth. So there is no difference between adding a fifth part to the number that has been calculated and taking away a 6th part, in this case 600 feet. There remains 500 which is the number of men held in a hemistriga.
36. But we have a force of 4000; we may see how many times 500 1 have; it is 8. This is the number of hemistriqae to be made. This makes 240 feet, and the assignment of the alae calculated above was 300 feet, making a total of 540. 3 cohorts can pitch their tents here which will take up 720 feet from which the amount the unit occupies is subtracted, that is 540 feet; there remains 180 feet in which 6 hemistrigae can be set up. Like the one above the 1st cohort, access roads are made in the same way for the units, each one being 4-10 feet – 60 feet is assigned for the roads. This leaves 120 feet which we assign to the tents of the tribunes and legates at the rate of 60 feet each.
37. Now if the surveyor assigns to the place 1000 extra men in the same area, we will have the following. Since half of this is 500, which is assigned to a hen-dstriq, we deduct 10 feet from the scamnum and build a road between the alae, which is given if the space permits. There will be 30 feet; this will be a hemistriga which will be assigned to the remaining 500 men.
38. Now on the opposite side in the same way we deduct from the total force 1000 men whose lines will be in a space of 60 feet. We will assign 80 feet to the scamnum of the legates, 70 feet to those of the tribunes and place roads between the alae.
Sides of the Praetorium and Retentura
39. It is the same for the sides of the praetorium and retentura, if there should be a greater or smaller force, and we also look out so that we increase or decrease the area of the praetorium and allocation to the emperor’s companions, and likewise for the quaestorium, so that the proportion of the width of the camp is retained. If, however, the space is more restricted, the roads between the Praetorian cohorts and cavalry units can be left out because if they observe military discipline, as I stated above, the soldiers will each gather in their own units.
40. In the retentura the men are accustomed to camp 50 per line closely or more widely, and since it happens that the units often change, they may have to pitch more tents although the lines run in the same area. No more should be taken away from these except quingenary infantry cohorts above the first cohorts and if there should be more units, but not enough to fill a line, it will be necessary to camp more closely in the remaining lines, as stated. Similarly it should be organized in such a way that they camp more spaciously whenever the remaining numbers permit so that the organisation of all the measurements is not thrown into confusion. So that they may camp in the lines of the retentura, in equal numbers, it is useful if the numbers in the retentura have already been calculated, as in the case of the praetentura.
41. If there should be more or fewer reinforcement troops than we have shown, everything changes and the cohorts camp round the rampart in a different way.
42. Now we have spoken about the 6820 troops in the half -part of the retentura. As this is 600 feet wide, I will see how many hemistriga there can be. In the present case this will be 17 and enough space can be assigned to the quaestorium. And so I take a 17th part of the force which is as we have said, the number of hemistriqae. This is 400, and so this will be the number of soldiers. They should camp in one hemistriga plus a fifth part of the length which is 80 feet; so this makes 480 feet from which it emerges that two cohorts camp on the side of the retentura.
Distribution of the Allies
43. We should distribute the allies and other tribes in the lines, but they should not be in more than three groups, nor far from each other so that they can hear the watchword in their own tongue more quickly. We should observe that the standards are assigned the first line, the same as those of the first cohorts, so that the access roads can run through the camp.
44. There will be 16 legionary cohorts along the sides, 6 in the praetentura and 4 in the retentura, each one being assigned 60 x 360 feet; the other four cohorts camp inside the via sagularis.
45. As far as I am able, brother Lord, on account of my inexperience of military affairs, I have followed briefly all the authors and whatever they wrote about the organization of summer camps, and I have explained everything in a logical system in this pamphlet before arranging the troops. To this day no author has explained in written instructions all the measuring from the beginning, and because of this I hope that our care will be rightly approved by you.
46. So we have explained the disposition of the legions and assigned the whole army to their places; we have also shown which units should be moved if necessary. If the alae should be placed in the retentura and infantry or partmounted cohorts in the praetentura, for no particular reason, it is without doubt a sign of an inexperienced surveyor. It is obvious that if there are no part-mounted cohorts at all in the army, we should place quingenary alae on the sides of the quaestorium so that the retentura has some cavalry.
47. It concerns the arrangement of the legions and the division of the units, it shows the difficulties even to those skilled in camp organization relating to the number of legions. So if you should condescend to use it I will be the first to carry to your magnanimity this new method of measuring which I hope will please you if you first examine the usual method of measuring.
Fortifications of Camps
48. Now we shall deal briefly with the fortifications of camps and other matters on which a number of authors have written. Five types of fortification of summer camps may be noted: ditch, rampart, stakes, stockade and weapons.
49. In a more secure place, the ditch is used for the sake of discipline, and the types are V-shaped or Punic. It is called V-shaped when the sides, sloping in from the top at the same angle and becoming narrower, reach the bottom. A ditch is Punic when the outer side is laid out vertical; the other side is inclined as in the V-shaped. They should be at least five feet wide and three feet deep. A similar ditch should be dug 60 feet in front of the gateway, and the same width as the gate. Because of its shortness, it is known as a titulum.
50. In less secure places a rampart of turf, stone, rocks or rubble should be thrown up. Eight feet wide and six feet wide will suffice, and a little parapet. There should also be a rampart before the gates along the titulum as along the ditches; because of the construction it is known as “sanctum”.
51. The stakes are trunks with their branches. They are resorted to if the nature of the soil is too friable causing the turfs to break, if a thick enough rampart cannot be built because of the looseness of the stones, or if a ditch cannot be dug without the sides collapsing.
Shortage of Stakes
52. When there is a shortage of stakes and the place is insecure, they protect the camp with four rows of armed men so that in each row guards are stationed frequently; cavalry should make a circuit of the camp by turns. If the camp is in a peaceful country, one row of arms will suffice for the sole purpose of maintaining discipline, and the guards are stationed more widely.
53. However, the same happens with the stockade as with the rampart if the place is rocky or sandy, because without doubt a stockade provides sufficient fortification for a camp.
The Angles of the Camp
54. The angles of the camps should be rounded because they make the projections and weaken the work which protects the defences. They should be rounded from the angle of the cohorts which make the sides of the work 60 feet and until the line re-joins the outside line of the rampart, and this makes an angle of 90′.
55. In the same way the clavicula is traced round a circle from a line on the inside of the rampart from a point in the middle of the gate, the compasses wide open to the edge of the gate; from this centre point you draw an arc in front of the road following the same line which is fixed at the centre of the gate. Then with the compasses in the same place you add the width of the rampart and draw another arc on the same line so that those going in are always unprotected and those coming in a straight line are kept out; and it gets the name clavicula. from this effect.
Choice of Terrain
56. Concerning the choice of terrain for the establishment of the camp; first they choose a site which rises gently above the plain, on a distinctive rise and the porta decumana is set at the highest point so that the area is dominated by the camp. The porta praetoria should always look towards the enemy. The second place is situated on a flat plain, the third is on a hill, the fourth on a mountain, the fifth in whatever place is necessary, from which it is called an “unavoidable camp”.
57. It should be particularly noted that a road should be built which is longer than the sides of the camp. Whatever the position of the camp there should be a river or spring on one side or the other. Unfavourable positions, which were called mothers-in-law by our ancestors, should be avoided at all times: the camp should not be overlooked by a mountain from which the enemy could attack or see what is going on in the camp; there should be no forest nearby to conceal a hidden enemy, nor gulleys or valleys through which the enemy may secretly approach the camp; nor should the camp be near a fast-flowing river which might flood and overwhelm the camp in a sudden storm.
58. In hostile territory one must remember to construct numerous double width access ramps up to the rampart and to build artillery platforms around the gates, on the projections at the corners and in places on towers. In particular the rampart should be fitted out with artillery on any side which is a mother-in-law if this cannot be avoided.