During the early-second century, for the first time in Britain, auxiliary cohorts began to be split into smaller detachments and housed in two (or more) purpose-built encampments. The camp which contained the commanding-officer's house and the regimental H.Q. are nowadays termed 'small forts', while the other type of camp without any administrative buildings are called 'fortlets'.

Small forts were also built to accommodate the officers and men of small auxiliary units called numeri (sing. numerus), and for this reason should be called 'numerus forts' in order to avoid confusion. The original Roman terms for all these smaller types of fortification are not known.

Locations of Fortlets in Roman Britain