An area excavation was undertaken in December 2004-May 2005 within the western part of the interior and defences of Metchley Roman fort, Birmingham (central England) in advance of proposals for a new hospital development. This was the largest single excavation of the fort interior undertaken at Metchley, comprising an area of approximately 0.4ha, equivalent to approximately 9% of the total internal area of the Phase 1 and Phase 3 fort. This volume, the fourth in the reports, concentrates on the researches into the western fort interior, defences, and post-Roman activity.