Roman Forts

The Roman word for a fort was castellum. The purpose of a fort was to house a military unit as a security force for a small district. The size of the unit housed in a fort would be too small to fight a pitched battle, but it was enough to mount a defence against raiders.

The fort in Roman Britain required strong defences against sudden attack and comfortable quarters suited to a semi-permanent occupation. Thus its defences were designed to be permanent and massive and its internal buildings durable and solid. Nearly all Roman forts were "playing-card" shaped, i.e. rectangular with rounded corners. The headquarters building was placed in the centre of the fort, with space for soldiers' accommodation in front and behind.

A castellum was designed to house either a cohors quingenaria of 480 men or a cohors militaria of 800 men, both of which might include mounted attachments. These were symbolised by the additional word equitata, e.g. cohors militaria equitata. The castellum might also house an ala quingenaria of 512 men or the much rarer and more specialised ala militaria of 768 men. A cohort was divided into six or ten centuriae of 80 men commanded by a centurion, whilst the alae were divided into 16 and 24 turmae, each containing 32 men and commanded by a decurion. 

It has been estimated that a fort 400ft square could contain a cohors quingenaria (480 men), with an extra 3,750 square feet being needed for a mounted attachment. A fort of 400ft x 600ft could hold a cohors militaria (800 men) or an ala quingenaria (512 men), or even a cohors militaria equitata. An ala militaria required a fort 500ft x 700ft, and forts of this size are peculiar to Britain.

Normally a fort was rectangular and surrounded by a huge rampart bank known as the vallum and one or more ditches (fossae). The fort would have been entered via one of four gates in the rampart bank. In the shorter sides of the rectangle these gates were usually placed in the centre, but on the longer sides to allow for room for the central row of main buildings they were pushed towards the front of the fort.

In the middle of the fort stood the headquarters or principia, whose main entrance faced the front gate of the fort (the porta praetoria), and to which it was connected by a road called the via praetoria. At a right angle to the via praetoria was the via principalis, which ran in front of the headquarters and connected the two side gates, the porta principalis dextra (right main gate) and theporta principalis sinistra (left main gate).

Either side of the headquarters would have been the other official buildings, including the house of the Commandant and one or more granaries. In some forts there would also be a workshop or smithy. This block of official buildings divided the front area (praetentura) from the rear area (retentura). In these areas the long narrow barrack buildings for the soldiers would be found.

Parallel to the via principalis and dividing the central block from the rear  was the via quintata. From the back of the headquarters the via decumana ran from the headquarters to the back gate, orporta decumana. A further road ran all the way round the fort within the rampart, and this was called the via sagularis.

[Original author: Miranda Greene, 2004]