Tomen y Mur is a Roman fort complex in Gwynedd, Wales. The fort was constructed under governor Gnaeus Julius Agricola in AD 78, and was abandoned around AD 140. A millennium later, in the Norman period, the site was reoccupied and refortified with a motte within the old walls. It is a scheduled monument in the care of Snowdonia National Park Authority. Tomen y Mur lies on the slope of an isolated spur above the A470, Northeast of Llyn Trawsfynydd. It is one of the most exposed and isolated Roman sites in Wales.