The Romans founded Chester as Deva Victrix in AD 70s in the land of the Celtic Cornovii, according to ancient cartographer Ptolemy, as a fortress during the Roman expansion north. It was named Deva either after the goddess of the Dee, or directly from the British name for the river. The 'victrix' part of the name was taken from the title of the Legio XX "Valeria Victrix" who were based at Deva. A civilian settlement grew around the settlement, probably starting as a group of traders and their families who were profiting from trade with the fortress. The fortress was 20% larger than other fortresses in Britannia built around the same time at York (Eboracum) and Caerleon (Isca Augusta); this has led to the suggestion that the fortress may have been intended to become the capital of the province rather than London (Londinium). The civilian amphitheatre which was built in 1st century could sit between 8,000 and 10,000 people, is the largest known military amphitheatre in Britain, and is also a Scheduled Monument. The Minerva Shrine in the Roman quarry is the only rock cut Roman shrine still in situ in Britain. The fortress was garrisoned by the legion until at least the late 4th century. Although the army would have abandoned the fortress by 410 when the Romans retreated from Britannia the civilians settlement continued and its occupants probably continued to use the fortress and its defences as protection from raiders in the Irish Sea.