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The dark blue shows the key structural features, including the large rectangular barrack block that takes up most of the western half of the site. The smaller rectangular building in the south-east corner is probably medieval, although we don't know whether it's pre- or post-Conquest (my money is on the latter, though I'd be agreeably surprised if it was earlier). The other stretches of walling along the eastern and northern edge of the site and the north-east corner are features built into the rampart of the fort and of Roman date (we think)- certainly the corner tower is Roman and was previously partially uncovered in excavations that took place on site in the 1930s.