A substantial, five-roomed masonry building in Hollytrees Meadow. A vaulted drain (MCC1515) joined the north-east corner of the largest room at floor level. This room was sunken and had several slots in the ground which may have been for timber partition walls or to hold machinery of some kind. The room also had a sump and a spring rose in the south-west corner. To the south of this room was a narrow compartment divided into three and then two further rooms. To the north was another room.

The building was first discovered by Dr. P. Duncan in 1852 during his excavations (ECC621) within Hollytrees Meadow.<1> These excavations were observed on several occasions by William Wire.<2> Further excavations were directed by P.G. Laver, and later Rex Hull, between 1927 and 1929 (ECC793 and ECC795) when three rooms were recorded. The building was once more exposed during excavations by Rex Hull in 1954 (ECC1760) when a further room was discovered, discounting his earlier theory that the building was a Mithraic temple (although the name has stuck). He concluded that the building had been deliberately emolished not earlier than the reign of Constans (AD 337-50). This excavation also confirmed that several walls discovered in 1928/9 were in fact parts of the outer walls of the building.<3>

Crummy has suggested the building may have been a waterworks which would have contained water-lifting gear. Iron shackles found in the building may indicate the use of slaves.<4>