Of this great royal and ecclesiastical centre, little is yet known archaeologically. Excavations on the Roman fortress walls have shown that they may have survived more or less intact for much of their circuit, and the Anglian Tower, a small square tower built to fill a gap in the Roman way, may be a repair of the Anglian period. The survival of the walls and gates shows that the Roman street pattern survived, at least in part, inside the fortress. Certainly excavations beneath York Minster have shown that the great hall of the Roman headquarters building still stood and was used until the 9th century.