Just east of the town centre, these low stone foundations represent the westernmost legionary fort of the Roman Empire. The fort dates back to AD 77, when General Gnaeus Julius Agricola completed the Roman conquest of Wales by capturing the Isle of Anglesey. Sadly the on-site museum is closed, and the only interpretive sign is on the side of the building. The site is about half a mile along the A4085 (to Beddgelert), which crosses through the middle of it.

Segontium was designed to accommodate a force of up to 1000 infantrymen, and coins recovered from the site indicate that it was an active garrison until AD 394 – a reflection of its crucial strategic position. Caernarfon's name is a reference to this site, meaning 'fort opposite Anglesey'.