Isca was of great strategic importance to the Romans, situated on a river where they could land men and supplies and from this strongly fortified base, march out into the mountains of Wales to strengthen their iron grip on the country. The fort began as a temporary wooden stockade protected by a ditch filled with water, but within a short time this was replaced by strong stone walls and barrack buildings for the legion, laid out according to standard Roman form.


By the early 3rd Century the garrison had been much reduced in size and from 260 AD the fortress went into gradual decline. As a key fort Isca was virtually obsolete by 350 AD for it had been replaced by a fort at Cardiff, which was built for protection against Irish raiders. The Second Legion was then marched away from Isca to man the great fort of the Saxon Shore at Richborough in Kent.