Half a mile uphill from the castle, on the southeastern edge of the modern town, the low stone footings of Segontium mark the site of a Roman auxiliary fort founded by Agricola around AD 77. It is by far the oldest visible monument in Caernarfon and the reason there is a town here at all — Edward I built his castle within sight of the ruins, conscious of the prestige Roman associations carried.
The visible remains are the foundations of barracks, the headquarters building, granaries, and a small bath-house, set out on a neat playing-card plan. A small museum interprets the finds, which include altars, coins, and a fragment of a temple of Mithras discovered nearby. Entry is free. Allow an hour.