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.

Tomen y Mur is the Welsh name for the site which means "mound of the wall"; as it is an allusion to Norman Motte that stands within the remains of the Roman embankments, hence cannot be earlier than the 11th century.

According to Tacitus, the native Celtic tribe of Ordovices aggressively resisted Roman occupation of their territory in central and Northwest Wales.

Shortly before Agricola's governorship began in AD 78, the Ordovices were still actively resisting and had massacred an entire regiment of Roman Cavalry based in their territory.

The rebuilding work suggests the military situation was stabilising and that legionaries had settled down into a garrison role, so much so that roughly 30 years later the fort was abandoned.