This sub-rectangular stone structure, built from a mix of limestone blocks, boulders and greywacke roughly arranged in courses, originally rose three storeys high, though the top floor has since been truncated and topped with a modern slate roof. The gateway features a flattened barrel-vaulted entrance with a segmental arch constructed from small greywacke slabs and a central keystone, creating an impressive passage that has welcomed visitors for generations.

The building’s interior reveals the complexity of medieval architecture and subsequent alterations over time. At ground level in the northeast corner, a small rectangular chamber measuring approximately 15 by 3 metres can be accessed through a flat-arched doorway, its barrel-vaulted ceiling still intact. The first floor, now reached by modern steps through a doorway in the north wall’s eastern end, would originally have been accessed via a spiral stairwell; remnants of which can still be spotted just above the ground-floor chamber entrance. Various architectural features tell the story of the building’s evolution: the north wall displays a later two-centred arch opening directly above the main entrance, whilst the south wall retains an original window jamb of hammer-dressed limestone, complete with glazing bar holes and chamfered edges.

Evidence of the Tholsel’s residential function survives in several practical medieval features. The southeast corner preserves the remains of a garderobe, complete with its small rectangular chute, whilst a plain slit opening in the south wall’s eastern end would have provided light to this medieval toilet. Plain lintelled loops pierce the west wall at both first and second-floor levels, with a cubby-hole tucked into the western corner at first-floor level and a single-splayed opening in the north wall. The stonework throughout shows considerable repair and alteration, particularly visible in the south wall of the eastern angle and the north wall of the northern angle, with some early medieval cut stone apparently incorporated from another building entirely, suggesting the Tholsel has been continuously maintained and adapted throughout its long history as Carlingford’s principal gateway.