Just like today, the army in Roman Britannia was organised into units of different sizes.  There were originally four main army units called 'legions' (each with about 5,000 soldiers) sent to Britain. The 2nd Augustan Legion - stationed at Isca Augusta (Caerleon) in South Wales. The 9th Spanish Legion - stationed at Eboracum (York) in Yorkshire, but they only stayed for about 70 years. They were replaced by the 6th Victorious Legion. The 14th Geminan Legion - only stayed for 20 years The 20th Victorious Valerian Legion - stationed at Deva Victrix (Chester) in Cheshire. Each legion was commanded by a legate. They were divided into 9 cohorts. Each cohort was made up of 6 centuries. A century had about 80 men commanded by a centurion. The legions sent out smaller units, called 'vexellations,' to where they were needed around the province. So there were Roman forts all over the place. There were also lots of cohorts of 'auxiliaries' (which means helpers). Lots of them were stationed on Hadrian's Wall.