The site of present-day Dorchester may have originally been a small garrison fort for the Legio II Augusta established shortly after the Roman conquest. When the military moved away, around AD 70, Durnovaria became a civilian settlement, apparently the "civitas Durotrigum" of the tribal confederacy of the Durotriges. Shafts were dug to deposit ritual foundation items. An organised street plan was laid out, ignoring earlier boundaries, the streets lined with timber-slot structures; public buildings including "thermae" were erected and an artificial water supply established. The town seems to have become one of twin capitals for the local Durotriges tribe. It was an important local market centre, particularly for Purbeck marble, shale and the pottery industries from Poole Harbour and the New Forest. The town remained small, around the central and southern areas of the present settlement, until expansion to the north-west, around Colliton Park, in the 2nd century. By the middle of this century, the town defences were added and Maumbury Rings, a neolithic henge monument, was converted for use as an amphitheatre. The third century saw the first replacement of timber buildings with stone ones, an unexpectedly late development in an area with several good sources of building stone. There were many fine homes for rich families and their excavated mosaic floors suggest a mosaic school of art had a workshop in the town, members of which seem to have travelled in the area to execute mosaic floors in villas away from Durnovaria itself. A large late-Roman and Christian cemetery has been excavated at Poundbury just to the west of the town, but little is known of Durnovaria's decline after the departure of the Roman administration. The name, however, survived to become the Anglo-Saxon "Dornwaraceaster" and modern 'Dorchester'.