Until the Roman conquest of Britain, Cardiff was part of the territory of the Silures – a Celtic British tribe that flourished in the Iron Age – whose territory included the areas that would become known as Breconshire, Monmouthshire and Glamorgan. The fort established by the Romans near the mouth of the River Taff in AD 75, in what would become the north western boundary of the centre of Cardiff, was built over an extensive settlement that had been established by the Romans in the 50s AD. The fort was one of a series of military outposts associated with "Isca Augusta" (Caerleon) that acted as border defences. The fort may have been abandoned in the early 2nd century as the area had been subdued. However, by this time a civilian settlement, or "vicus", was established. It was likely made up of traders who made a living from the fort, ex-soldiers and their families. A Roman villa has been discovered at Ely. Contemporary with the Saxon Shore forts of the 3rd and 4th centuries, a stone fortress was established at Cardiff. Similar to the shore forts, the fortress was built to protect Britannia from raiders. Coins from the reign of Gratian indicate that Cardiff was inhabited until at least the 4th century; the fort was abandoned towards the end of the 4th century, as the last Roman legions left the province of Britannia with Magnus Maximus.