In May and June 1993 The Canterbury Archaeological Trust excavated within the former gymnasium of the King's School prior to the insertion of foundations, services and flooring for a new theatre and the sitecode was SMN93.
The earliest features exposed were of Roman date and included a length of the town wall constructed c. 270-90 AD. It was made of flints bonded in mortar and stood to a height of 1m extending parallel to and below the north wall of the gynasium. The remains of the earthern rampart were also seen to the rear of the wall. Capping the rampart was a 0.7m thick deposit of loam thought to have developed when the rampart was reduced/removed, possibly at an early date, prior to the 12th century and during a period of abandonment.
A flint wall foundation was located 8m to the south of the town wall and was thought to date to 12th century being the priory boundary wall. Between the walls was found traces of metalling, possibly from a lane dating to after the construction of the wall.
The remains of chalk walls were found to the rear of the city wall, thought to date to around the 13-14th centuries and representing an extension to St Mary's Church to form a new chancel south east of the North Gate. Evidence was also seen for the rebuilding of the city wall in c. 1390-6 AD with chalk blocks surmounting the Roman wall and it was then that this extension may have been built. In the south east corner of the excavation a building, thought to be contemporary with the rebuild and extension, was uncovered consisting of chalk build on gravel footings.
Medieval Northgate was demolished in 1830 though the excavated evidence suggested that the south eastern chancel extension may have survived demolition. A number of later walls within the building together with small outhouse foundations to the south east suggest reuse as a dwelling. In 1859 the cottages against the Borough were demolished and it is likely that this is when the chancel extension was finally removed. The present range of building on the site were constrcuted in 1903.

Ward, A. & Bennett, P., 1995, St Mary Northgate (Article in serial). SKE30262.