The Brigantes were the major Celtic tribe in what is now known as Northern England; they had a stronghold in the locality at a sandstone outcrop on which Manchester Cathedral now stands, opposite the banks of the River Irwell. Their territory extended across the fertile lowland of what is now Salford and Stretford. Following the Roman conquest of Britain in the 1st century, General Agricola ordered the construction of a fort named Mamucium in the year 79 to ensure that Roman interests in Deva Victrix (Chester) and Eboracum (York) were protected from the Brigantes. Central Manchester has been permanently settled since this time. A stabilised fragment of foundations of the final version of the Roman fort is visible in Castlefield. The Roman habitation of Manchester probably ended around the 3rd century; its civilian settlement appears to have been abandoned by the mid-3rd century, although the fort may have supported a small garrison until the late 3rd or early 4th century. After the Roman withdrawal and Saxon conquest, the focus of settlement shifted to the confluence of the Irwell and Irk sometime before the arrival of the Normans after 1066. Much of the wider area was laid waste in the subsequent Harrying of the North.