In Theobald's oringinal priory structure, it is thought that the space between the Prior's Lodge and the west end of the church either contained a gatehouse or a simple passage giving access into the inner sactum of the claustral complex. All evidence of this structure was removed however when the north-west tower of the church was added in the late 14th to early 15th century.

The tower added to the north-west corner of the church, within the area thought originally to have been occupied by a gatehouse, was built to serve two purposes. First it was structural; the north wall of the church required the addition of a large buttress running along its length and the tower provided support for the western end of the same wall. It also supported the south wall of the Prior's Lodge, built upon the line of the earlier Lanfranc church foundations and possibly requiring additional stability. Secondly, the tower enhanced the appearance of the church frontage.

The tower, built over two storeys, measured c. 2.90m N-S by c. 3.30m E-W, with a passageway at ground level leading into the claustral complex, entered by a doorway in the western wall. The east wall probably contained a more open archway. The upper level of the tower may have accommodated the archbishop's treasury. To insert the tower, the south wall of the Prior's Lodge was demolished and the foundations were cut through by the much wider foundations of the tower's north wall, which were some 1.20m deep and 2.05m wide. The tower was demolished along with the rest of the church at the Dissolution.

Hicks, M., 1989, 'St. Gregory's Priory: The Church and Conventual Buildings' (Article in serial). SKE30366.

Hicks, A. & Hicks, M., 2001, St. Gregory's Priory, Northgate, Canterbury, Excavations 1988-1991 (Monograph). SKE30367.