A large defensive ditch on Cranborne Chase in the north east of the county, Bokerley Dyke, dated to 367, delayed the Saxon conquest of Dorset, with the Romano-British remaining in Dorset for 200 years after the withdrawal of the Roman legions. The inhospitable coastline prevented an invasion from the sea. When the Roman road across Cranborne Chase was rebuilt in the 6th century the Saxons advanced into Dorset. The Romano-British retreated, constructing another defence, Combs Ditch, which also fell within a century. It is thought that the Saxons captured east Dorset in the mid-7th century and west Dorset shortly afterwards, but the details are not known.