The Brigantes were the major Celtic tribe in what is now 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.[14] 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.[14] Central Manchester has been permanently settled since this time.[15] 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.[16] 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.[17] Much of the wider area was laid waste in the subsequent Harrying of the North.[18][19]