What's left of the Roman garrison town of Corstopitum lies about half a mile west of Market Pl on Dere St, once the main road from York to Scotland. It's the oldest fortified site in the area, predating the wall itself by some 40 years. Most of what you see here, though, dates from around AD 200, when the fort had developed into a civilian settlement and was the main base along the wall.
You get a sense of the domestic heart of the town from the visible remains. Revamped in 2018, the superb museum here displays artefacts unearthed at the site including Roman sculpture and carvings, including the amazing 3rd-century Corbridge Lion.