It was located 137 metres north-east of the Roman castrum of Segontium on the outskirts of modern Caernarfon in Gwynedd, Wales.
The remains were discovered by accident on 2 April 1958 and excavated by the National Museum of Wales in August of the following year under the direction of George Boon.
[1] The site was found to be already damaged by a sewer trench which cut across the anteroom and had removed part of the southeast corner, but the majority of the temple could be excavated.
The excavators noted the site was quite marshy and this suggested that a stream had once flowed close to the temple at the bottom of the shallow valley.
The building measured 14.6m by 6.55 and is tentatively dated to the third century AD, a period when the fort was occupied by the Cohors I Sunicorum.
The fact that each base is different (including one circular in shape while the rest are all square) suggested to the excavators that they were salvaged from various other buildings.
This indicated a period of abandonment prior to the burning of the roof and also explains why no Mithraic sculptures were found in the building, as they had all been removed.