Segontium Roman Fort, said to have been the city Macsen Wledig saw in his dream, is situated in the heart of Caernarfon on Ffordd Cwstenin (Constantine Road). It was one of the largest Roman forts in North Wales, and, being occupied from around 77AD to 395AD, was in use for longer than any other Roman fort in Wales.

This suggests that it had a role in administration, the organisation of mining operations and in the collection of taxes. Other Roman Forts in Gwynedd include Aberffraw, Pen Llystyn at Bryncir, Canovium (Caerhun), Bryn-y-Gefeiliau (Caer Llugwy), Tomen y Mur, Brithdir, Caer Gai and Cefn Caer - Pennal.

Cn. Julius Agricola became governor of Britain in 77AD and was immediately faced with, and crushed, a rebellion by the tribe of Ordovices who lived in hillforts in central Wales. They had recently almost wiped out a regiment of Roman cavalry stationed in their territory. Segontium was founded by Agricola, soon after.

The fort was originally built of wood with a rectangular bank and ditch enclosure and a wooden palisade. It was designed to accommodate 1000 men. It was rebuilt of stone around 140AD.

After this there were frequent programmes of rebuilding, perhaps due to repeated attacks or the desire to make improvements.

The site has been extensively excavated over the last 160 years. The footings of the buildings in the centre of the site have been left exposed so visitors can gain an understanding of the site.

On the southside of the road is a bathhouse complex. The finds from the excavations can be seen in the museum at the site.

Excavations have uncovered two bathhouses at Segontium, which were enclosed by buildings presumably used by the elite of the fort. Although it was common in Roman society for most people to use baths, at Segontium the soldiers had no access to them. The two bathhouses were not in use at the same time.

The earliest bathhouse seems to have been built early in the 2nd-century and continued in use until the late third century when the bathhouse that can be seen today was built to replace it. It is probable that the later bathhouse was never completed as no evidence of a hypocaust or drainage was found. Both bathhouses contained a frigidarium, or cold room, containing a cold plunge bath in the apse; a tepidarium, or warm room, with a dry, warm atmosphere, where bathers could undress, take gentle exercise and oil themselves ready for the caldarium, or hot room, heated to temperatures of 40 degrees Celsius and containing a hot plunge bath.

To heat the bath hot air from a furnace was pumped beneath the floor raised on pillars of square tiles (the hypocaust). Hot air also came through the walls in the caldarium to create a sauna affect.

A Mithraic temple was found situated on what is now Lon Arfon, 170m east of the walls of the fort. It was excavated in 1958 in preparation for building houses. It differed in style from the buildings inside the fort as it was constructed of beach pebbles and roofed with slate, while they were built of brick and stone with tile roofs. This suggests that it was not built with the help of the state, but through personal devotion. The temple was built and used in the 3rd-century, at the time when Mithraism was in its most popular era. The temple measured 16m long by 7m wide divided into a narthex at the back, a nave in the middle with an aisle on each side and a niche at the front where the statue or relief of Mithras would have been positioned.

No artefacts associated with Mithraism were found in the excavation, but other Mithraic temples of exactly the same design have been found elsewhere in Britain and throughout the empire. A candlestick, terracotta lamps, and a metal cup were amongst the finds as well as pottery and coins. It was abandoned in the 4th century and was destroyed soon afterwards. It is possible that the temple was ruined when Christianity was established.

The barracks in the fort each accommodated 80 infantrymen or 64 cavalrymen. The other buildings included storehouses and workshops. As is shown by the position of the temple, the settlement connecting to the fort extended beyond the walls of the fort and housed families and traders. Most of the evidence for the wider settlement is inaccessible as it is under housing estates.