The Gatehouse, including attached section of town wall at the rear

The Gatehouse is a 2½-storey house dating from the 18th century, with an attached section of medieval town wall at the rear. The front of the house is pebble-dashed, with smooth-rendered quoins, and has a steep slate roof, which is grouted to the rear slope. A brick stack stands against the adjacent No. 50 Castle Street, and another stack is located on the rear wing to the left (west) side of the house. The right-hand corner is rounded.

The entrance is offset to the right of centre and is framed by a panelled door within a reed-moulded doorcase topped with a pediment. Windows have smooth-rendered architraves. To the left of the entrance is a four-pane horizontal-sliding sash window, and to its right is a later, nine-pane margin-lit window. The upper storey features two two-light casements, replacing earlier sash windows, and two flat-roofed dormers with two-light windows.

At the rear, a late 20th century single-storey projection extends from the main house, with a balcony above. The rear wing has a lower ridge line and was extended in the 20th century. Both original and later sections have replacement upper-storey windows. A small casement sits within a flat roof dormer to the left of centre on the main range, with a larger added dormer also with a flat roof positioned to its left.

Attached to the east side of the rear is a short section of the former medieval town wall, constructed from rubble stone, approximately 6 metres long and 5 metres high.

The main range now comprises a single room with a joist-beam ceiling and three cross beams, all of which are chamfered with run-out stops. A large timber lintel is above the fireplace to the left of the entrance. The original back door, now located within the added single-storey projection, has chamfered stone jambs—possibly salvaged from an earlier gatehouse near the site—and a stone lintel of a different character. The dog-leg staircase has a square newel and some 18th-century fretwork balusters, but has otherwise been entirely rebuilt. An earlier stair window was blocked, likely when No. 50 Castle Street was built. The upper storey has two cross beams, and a corbel against the left gable end which may also be salvaged from the earlier gatehouse. The roof displays one collar-beam truss with raking struts. Original purlins were removed and replaced when the eaves were slightly raised, resulting in a roof with a flatter pitch. The rear wing, in the lower storey, features a single spine beam and a rough timber lintel over the fireplace.