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| Tomen y Mur | |
|---|---|
| Gwynedd, Wales, UK | |
 Reconstructed wall at the northwestern corner of the fort | |
  Tomen y Mur Location in Gwynedd | |
| Coordinates | 52°55′48″N 3°55′36″W / 52.9299°N 3.9268°W |
| Grid reference | SH705386 |
Tomen y Mur is a First Century AD Roman fort in Snowdonia, Gwynedd, Wales. The fortification, which lies on the slope of an isolated spur northeast of Llyn Trawsfynydd, was constructed during the North Wales campaigns of governor Gnaeus Julius Agricola in AD 78. The fort stood adjacent to the Roman road known as Sarn Helen. It was occupied until it was abandoned around AD 140. In the 11th century, the Normans reused part of the site for a motte.
Tomen y Mur, which is managed by Snowdonia National Park Authority, is a scheduled monument.