When Hadrian's wall was built in 122 A.D., three outpost were constructed north of the defences for scouting and intelligence. Bewcastle's fort was probably known as Fanum Cocidii, adopting the local pagan god Cocidius. Built at first from turf and timber, it was unusual in having six sides. The garrison was a cohort, 1000 men, the First Nervan Cohort of Germans.

A castle was built soon after 1092, using part of the Roman ditch system and probably constructed of timber. The present stone castle was built, using material from the Roman fort, by one of Edward III's generals, John de Strivelyn between 1340 & 1360. Falling into disrepair under the de Middleton family, it was thought to have been repaired by the Warden of the English West March.