The Roman fortification, whether it was a temporary overnight camp in enemy territory, an auxiliary outpost fort set to guard a strategic location, or a large fortress to garrison the might of the Roman legions, was almost invariably built to the same basic formula. Although Roman camps have been found of various shapes, by far the most common form is that of a quadrilateral with parallel sides and rounded corners. The outline of the fortifications can generally be used as a rough guide to the period in which they were built. The traditional ‘playing-card’ shape is typical of the first century, whereas by the second and third centuries, squat, square-shaped forts predominate. By the fourth century the Roman military began to depart from this strict, angular form, as attested by the ovular outline of the Saxon Shore Fort at Pevensey (Anderitum).

A Roman camp was always enclosed by a defensive system comprising at least three components;

1. At least one ditch or fosse.
2. An inner rampart or agger containing the ditch outcast.
3. A palisade or vallum surmounting the rampart.

Within the defences, the camp was divided into three main areas:

• Praetorium – Situated at the geometric centre of the camp, this area was where the commander’s tent was pitched and the military standards grounded. The areas to left and right of the praetorium outwards up to the defences of the camp (marked D[extra] and S[inistra] on the accompanying plan) were occupied by the various military stores, hospitals, workshops and the like. In a temporary encampment in enemy territory, this area would house the grain and military supplies along with the carts and baggage animals needed to haul it. Any artillery equipment such as ballistae or onagri which was carted by the unit, may have been stored in this central area when not emplaced along the defences, as urgency and need dictated.
• Praetentura – This area was the forward part of the camp closest to the enemy, where the cream of the Roman soldiers themselves were barracked. In a legionary camp this was where the first cohort was encamped, together with the strongest of the other cohorts.
• Retentura – This was the rearward part of the camp which housed the remaining cohorts; roughly half of the military force. If there were any mounted troops (i.e. the cavalry contingent attached to a Roman legion) they were generally placed in this area.

The areas within the camp were delimited by a number of roads:

• Via Sagularis – Known also as the ‘intervallum road’, this ran round the complete circuit of the camp within the defences.
• Via Praetoria – Led from the praetorium in the centre of the camp toward the enemy, bisecting the praetentura. The point at which this road pierced the defences marked the front gate of the camp.
• Via Decumana – This road ran from behind the praetorium in the centre of the camp towards the rear gate, bisecting the retentura.
• Via Principalis – This road ran in front of the praetorium in the centre of the camp at right angles to the via praetoria, separating the praetorium from the praetentura in the forward part of the camp. This road was usually continued at both ends through gateways in the defensive circuit.
• Via Quintana – This road ran at right angles to the via decumana, separating the praetorium from the retentura at the rear of the camp. This road often terminated at the via sagularis, without gates through the defensive circuit, unlike the via principalis.

The defences were pierced by gateways, usually four, at the termini of the major internal roads:

• Porta Praetoria – The main entrance to the camp at the terminus of the via praetoria, was invariably placed in the centre of the defenses facing the direction from which any potential danger would arise, e.g. a strategic river-crossing, an enemy stronghold, etc.
• Porta Decumana – Was situated at the rear of the camp, opposite the praetorian gate, at the terminus of the via decumana. In some of the smaller forts, and a fair proportion of fortlets, this gate was not present.
• Portae Principali – These gates were situated at the sides of the encampment, usually placed forward of the central point of the defences, at either end of the via principalis; the gate on the right side of the camp being named the porta principalis dextra, the left-hand gate was, unsurprisingly, named the porta principalis sinistra.
• Portae Quintanae – These gates were often omitted, especially in the smaller encampments. If present, they were situated in the sides of the defensive circuit, again off-set from the centre, this time towards the rear, at either end of the via quintana.

The dimensions and complexity of these defensive elements would obviously depend on the size of the force the camp was intended to house and the period of time that the encampment was planned to be occupied.

Construction Methods

The Marching Camp

In a temporary overnight camp not intended for re-use after the force had moved on the following morning, the agger may be formed merely from the piled-up outcast from the fosse, surmounted with a rough palisade of stakes thrust into the earth along the top of this bank. The overall width and size would vary depending on the number of men available for the task.

The Roman marching camp was constructed in the following manner:

1. The area would be scouted and the best site chosen.
2. The centre of the site would be marked by a flag; this would preferably be placed at a point slightly higher than the surrounding topography.
3. The camp engineer would take sightings using a single groma – a simple instrument which allowed the efficient sighting of right-angles – placed at the designated centre, and the positions of the intended gateways would be marked by other pairs of marker flags at measured distances paced out from this central point.
4. Upon the arrival at the camp site of the bulk of the force, each unit would move to its assigned position within the marked-out area and would dump its gear. The strongest and most experienced centuries would be first, and they would march through almost the entire length of the marked out area before turning aside and making camp; in this way the most experienced troops were set to work on the defences nearest to the enemy.
5. Every eight-man contubernium in each century would assign each of its members to different tasks:
   • If the camp was made in hostile territory, a proportion of the force would be used to form a defensive cordon around the remainder, who would prepare the encampment.
   • The bulk of the force would be used to construct the camp defenses, usually comprising of a single ditch and an inner bank formed from the ditch outcast, with a row of staves implanted in the top of the bank. If there were sufficient men, the defenses may be more elaborate, perhaps built of stacked turves.
   • Whilst the heavy construction fell to the rank and file, under the watchful eye of their centurions, some legionaries were excused the dirty work and as a consequence were termed immunes (Latin immunis, free or exempt from…). These would be required to perform the less arduous tasks; clearing the camp interior, unloading baggage, erecting tents, cooking dinner, tending horses, etc.
6. The first half of the force would already be employed building the forward part of the camp by the time the commander arrived and took up position at the centre. He would probably begin with a meeting of all centurions and officers to discuss any immediate defensive problems.
7. During the time that it took the rearward half of the force to reach the encampment, most of the defensive circuit would already have been delineated by a bank and ditch.

The Auxiliary Fort or Campaign Fort

A tactical auxiliary fort, built to last at least one campaign season guarding an important site, were built with more intrinsic strength and solidity. The usual method from Caesar’s time and throughout the 1st century A.D. was to build a rampart of turf retaining walls, but by the 2nd century the preferred method was to replace the front facing or both retaining walls in stone:

1. The site was surveyed using the same basic methods used for a marching camp, described above, though probably with less haste, on a more carefully chosen site.
2. After accurately marking out the positions of the ditches, ramparts and gates, work would begin by removing the turf from the areas delineating the ditches and ramparts.
3. The gathered turves would be stacked together forming two walls of turf (or stone), each about a yard (0.9 metre) thick and separated by a gap of around ten feet (3 metres).
4. While the turf walls were being stacked, post-holes would be dug at the corners and gaps in the ramparts, and choice timbers would be erected to form the frameworks of corner-towers and gateways.
5. The outcast from the ditches surrounding the fort would provide the infill material for the interior space between the turf walls. The processes of infilling and building-up the enclosing turf walls would be going on simultaneously.
6. The rampart may be strengthened or modified in a number of ways:
   • If sufficient local timber was available – as was often the case in Britain – the walls of the fort may be strengthened by the inclusion of a timber lattice-work within the intermural space.
   • If timber resources were plentiful the entire front of the rampart may be faced with timber, perhaps part-replacing the outer turf wall.
   • If the area was particulaly damp or prone to flooding, the entire rampart may be built upon a raft of logs or stones, again, if locally avaialable.
   • In some particularly damp areas, the rampart would perhaps be built of alternate laminated layers of sand and clay. Clay was also used to line ditches in sandy soil, to prevent slippage.
7. A turf rampart would be built almost vertically upwards for about ten feet, and would be between twelve to sixteen feet wide at its base. The top of the rampart could be anything between ten to four feet in width, depending on the angle of slope imparted to the turf walls at the front and rear.
8. The top surface of the rampart was boarded over by a catwalk, and a timber palisade erected at the front.
9. Turves for the defences were also removed from the areas delineating the major roads in the interior of the fort, which were then surfaced with gravel.
10. The interior buildings would be the last things constructed, mainly of timber, though sometimes in stone, particularly the sacellum in the centre of the fort, which housed the regimental standards and the treasury; in many Roman forts, the sacellum was the only building made of stone.