Castellated structure to NW of The Sluice

The building is a tall square castellated structure made of rubble stone, located to the northwest of The Sluice. It features a square plan, a castellated parapet, and tooled stone quoins. The east elevation, which faces The Sluice, has a high wide arched recess on the left and an arched doorway on the right with double doors. To the left of the recess is a small lunette light that illuminates a mural stair. Both the recess and the doorway are framed in square-headed designs, with tooled stone voussoirs and keystones that are slightly set back. Inside the recess, there is a cobbled floor, a cemented seat, and an arched door to the left leading to a stair within the wall, also featuring stone voussoirs.

On the north elevation facing Duke of York's Wharf, there are two tall blocked round-headed arches, which are the ends of the vaults inside, each with keystones and a stone pier between them, as well as an impost band. Above the arches, there are three small square blank openings in the spandrels, and a date stone from 1848 is positioned between the arches. The infill of each arch includes two windows with timber lintels.

The building contains two large stone-vaulted chambers.