Tomen y Mur is located in southern Snowdonia near Bala. Tomen y Mur, which means Mount in the Walls.

The site is a mashup of Roman, Norman, and Welsh settlement and is one of those special places whose history covers thousands of years, beginning prior to the Roman Conquest. Welsh mythology references the place with a story of Lleu and Blodeuwedd in the Mabinogion.

Starting in 78 AD, the Romans built walls, a fort, barracks, baths, parade ground, and small amphitheater—one of the few for just military personnel but an indication of the hardship of the posting, all to counter the Ordovices, who’d wiped out a Roman legion in the 50s. The Roman response was to move into the area in force and almost eliminate the entire people.

Tomen y Mur was built to oversee the remaining few and protect the crossroads of a particularly lawless area (from the Roman perspective) of Wales. The Welsh return to the site after the Romans left, and the earthworks date from that time.

The Normans arrived in the 1090s and threw up the earthen motte that still exists today, one of many such motte and bailey castles built in the era of Robert of Rhuddlan The last mention of Tomen y Mur is in 1114 when Henry I stopped there in his aborted attempt to conquer this portion of Wales.

Site continued to be used as a farm and the rail line for the slate mines cut right through the middle of the ampitheatre

The motte itself is a near-circular steep-sided mound, about 36m in diameter and 6.5m high with an embanked ditch runs around the foot of the mound. It was built of stone blocks taken from the fort ruins and may conceal a standing Roman gate. Many Roman ruins remain as well, including the ampitheatre and old road.