Archaeological excavation was carried out at the Mercury Theatre, Balkerne Gate, Colchester, Essex during the Mercury Rising project to extend the theatre. Located within the south-west corner of the Roman fortress and within Insula 25a of the later walled Roman town, previous archaeological investigations in and around the development site had confirmed the existence of: a 1st-century military plinth relating to the Roman legionary fortress; other pre-Boudiccan buildings/deposits; metalled street surfaces surrounding Insula 25a of the later walled town; and one or more Roman town-houses with in situ wall foundations, mosaics and tessellated floors. Two areas were excavated at 498 square metres (Area A) and 133 square metres (Area B). The earliest discoveries were military plinths from the early Roman fortress. At 0.6m wide and made of pebbles set in a creamy-brown mortar, similar plinths from across Colchester have previously been identified as the remains of barrack blocks. Plans of a 'typical' barracks, as excavated at Lion Walk and the Gilberd School in the 1970s/80s, were laid-out over the development site, and the plinths from the Mercury Theatre (including those from previous investigations) were found to align. This revealed, for the first time, that a group of six barrack blocks, similar to those previously identified to both the north and south of the development site, existed at this location in the fortress. There was evidence of reuse of the barracks in the colonia, and it is clear that they were still standing in AD60/61 as they were destroyed during the Boudiccan revolt. By the late 2nd century at least one but probably two town-houses had been built within Insula 25a of the walled Roman town. Nineteen rooms were uncovered, defined by either medieval robber trenches or more rarely in situ wall foundations. Twelve of the rooms had in situ tessellated floors. There was also one mortar floor, one of opus signinum, and another room had both a tessellated and sandy-clay floor. Building debris over the town-houses after their abandonment included fragments of imbrex and tegula from tiled roofs, with the remains of an imbrex stack attesting to the salvage of materials for collection and reuse. Large quantities of painted wall plaster were also recovered from the floors, likely fallen from the walls during a long period of dilapidation and decay after the roofs had been removed. Whatever was left of the town-house walls/walls foundations were robbed out in the medieval period. Anglo-Saxon finds included a small sherd of Saxon pottery (5th-7th century), a sceatta (c AD 680-710) and a finger-ring (9th-10th century). These finds add to the small but significant evidence for an Anglo-Saxon presence in this part of Colchester. Producing a radiocarbon date of 890 to 1160 calAD the inhumation of an adult female could be of Anglo-Saxon or early medieval date. A layer of medieval cultivation soil covered large parts of the site, but a series of beaten clay floors and hearths were revealed on the south edge of Area A representing a period of 13th- to 14th-century occupation. This appears to coincide with a significant phase of robbing of the wall foundations of the Roman town-houses. Pottery, a stone mortar, iron candlestick and copper-alloy buckle were included amongst the medieval finds assemblage. Two post-medieval inhumation burials were presumably buried within the churchyard at St Mary's at the Walls, and musket balls date to the Siege of Colchester. The foundations of both St Mary's Rectory (built 1873) and Mercury House (built 1968) were uncovered, with a later 19th-century beaten clay floor and hearth probably coming from a hut or shed within the rectory garden. Monitoring carried out during subsequent groundworks for services and landscaping identified one of the two post-medieval burials, and a medieval robber trench with in situ Roman wall foundation further to the east of Area A but likely from the same Roman town-house. None of the service trenches impacted on significant archaeological remains, and many of the groundworks in the north-west corner of the development site were excavated through the backfill of an early 19th-century reservoir.