The Castle Ditch is a large defensive ditch contemporary with Canterbury Castle. It was excavated when the castle itself was built c. 1180. It extends around the west, north and east sides of the castle, whilst the then existing city wall and defensive ditch provided the southern outer defences.

From summer 1976 to Autumn 1977 the Canterbury Archaeological Trust excavated within the Rosemary Lane Car Park in advance of redevelopment of the site. The sitecode was RLCP76/77. The site included the north west quarter of the outer bailey of Canterbury Castle. Three areas were looked at, area I in the outer bailey, area II to the north of this and outside the castle enclosure and area III to the east within the bailey. An additional small trench was also cut prior to the construction of a new building to the south of Gas Lane, in the Autumn of 1975.

The construction of the gasworks in the 1820s, together with the later extensions, some built as recently as 1953, extensively damaged and polluted the archaeological levels. Only in area II was intact stratigraphy found, elsewhere only deeply cut features survived. These archaeological features became sumps for tar and other noxious by-products of the manufacture of coal gas, which had soaked into the ground. The excavation of Area III was eventually abandoned due to this pollution. Early levels related to the Roman period.

No trace of Norman occupation was found apart from the castle ditch; this was 2.8m deep and 8.9m wide. The clearance of the area prior to the construction of the castle is suggested by a very mixed layer found in the small 1975 trench south of Gas Lane. This was sealed by a layer of plough soil, indicating that the area outside the castle was probably used for agricultural purposes for a considerable period after the castle was built. No trace of the bailey wall was discernable within the excavated area, and were probably removed during the construction of the gasworks. The junction between the bailey wall and the Roman city wall can also still be recognised by a change in wall construction, where the coursed mortared whole flints a the back of the Roman are interrupted by the closely set nineteenth-century knapped flints of the repair following the demolition of the bailey wall.

Bennett, P., Frere, S., Stowe, S., 1982, Excavations at Canterbury Castle (Monograph). SKE29766.