Antrim Castle Gatehouse, Market Square, Antrim, Co Antrim

This early 19th-century gatehouse, built in 1818 for Lord Massereene as the main entrance to Antrim Castle demesne, is designed in castle style and now serves as an important focal point in Antrim town centre, despite being cut off from the demesne by a modern road. Although the architect's name is not recorded, the design has been attributed to John Bowden of Dublin, though Bowden was only consulted on suggestions for improvement of a design already prepared by someone else. The building retains most of its original features and, despite the spoiling of its original setting, still forms part of an interesting group with the adjacent bastion and ramparts, providing the centre of town with an important and romantic feature.

The gatehouse is a crenellated, 2-storey structure of rectangular plan with octagonal stair turrets flanking each side of the entrance front. The walling is of snecked blackstone with moulded sandstone copings to crenellated parapets, moulded sandstone corbel course, moulded plinth, and sandstone dressings to openings. The main entrance faces north and comprises a wide Tudor arch with square drip moulding, containing three moulded shields bearing coats of arms: the Clotworthy Arms in the left-hand spandrel, the Skeffington Arms in the right-hand spandrel, and the combined Clotworthy and Skeffington Arms at the apex of the archway. The archway contains a pair of recessed arched timber doors, heavily studded in timber to give the appearance of iron bolt heads. Above this, at first floor level, is a rectangular mullioned 4-light window, unglazed, recessed in a chamfered square surround with a Tudor-style drip moulding.

Flanking each side of the archway and rising above parapet level are projecting octagonal towers with projecting plinths and oversailing parapets with crenellated tops. The main faces of the towers are pierced by narrow blind slit rectangular openings. The angles between gateway and towers are occupied by quadrants of replacement steel railings with ball finials; the original iron railings were removed and replaced in October 2004. The tower roofs are surmounted by ball finials carrying ironwork finials, supported by an open iron framework which was originally covered with slates. The main roof is flat, covered with asphalt behind the parapets.

The east elevation has walling similar to the north front, with one first floor window: a rectangular mullioned 2-light, unglazed, recessed in a moulded square surround. The rear elevation is 1-bay, 2-storey with a central Tudor archway open, without gates, a mullioned 3-light window to first floor, and a crenellated parapet. The walling here is of rubble basalt. Projecting forward from each extremity of the archway are storey-height basalt screen walls. A cast iron hopper and downpipe are attached to the rear of the eastern tower.

The west elevation is similar to the east side, though the ground floor is not visible as it is abutted by the adjacent bastion walling. The first floor walling is of basalt rubble with crenellated parapet and moulded corbel course. One small window opening, now blocked, is set in a moulded surround. In the angle with the tower is a small hipped-roofed basalt rubble walled outshot chamber supported on an iron girder, containing a 2-light timber window in a square surround, now lacking glazing; the roof is covered with iron plates.

The building stands in a corner of the market square facing directly onto the roadway. To the west of the entrance front, extending and then returning forward, is the stone wall of the bastion and wall-walk at a higher level, with a grassed area in front. To the east is a stone screen wall containing a Tudor archway, dating from 1852–6 when the adjacent bridewell and police station were built, linked to the front boundary wall of the former bridewell on the adjacent plot. This archway leads into a path to the rear of the gatehouse and beyond to the riverbank to the south. The main gatehouse archway leads to a former vehicular driveway to the castle but now functions as a pedestrian path. Behind the gateway are modern steps leading up to the former bastion and wall-walk.

Projecting from the east end of the south elevation is a basalt rubble wall running southwards, which has a crenellated top and one original semi-circular arched doorway, with three later Tudor arched doorways. This wall terminates in an angled turret near the river; this wall appears to be contemporary with the gatehouse but contains the three modern doorway openings.

The gatehouse has been misnamed a "barbican" since as early as the 1830s, when it was so referred to in the Ordnance Survey Memoirs. At one time the ground in front was separated from the rest of the town square by a line of iron railings on a low stone plinth, projecting northwards from the adjoining archway on the east side. Prior to that, the line of railings appears to have been set further back, skirting the adjacent ramparts to the west. The building stands within the area of ancient monument no. ANT 163.