Castle Square Presbyterian Church of Wales, including forecourt walls and railings

Castle Square Presbyterian Church of Wales is a Gothic style chapel built from coursed rock-faced yellow sandstone with a slate roof behind coped gables. The building is set back behind a forecourt enclosed by gates and railings.

The asymmetrical front elevation features a tower and spire positioned over a porch to the right, and a two-storey porch with a steep hipped roof to the left. In the centre, four lancet windows with ringed shafts and foliage capitals light the vestibule, while taller triple lancets with clustered shafts and foliage capitals light the main chapel above. An arcaded frieze and a cusped roundel to the gable complete the centrepiece. The left side is punctuated by an angle buttress with a polygonal turret, and the left-hand porch has an angle buttress and a quatrefoil frieze at the eaves.

The left porch contains a doorway with a boarded door featuring prominent false strap hinges, with a cusped circular light in the tympanum. Above this are three stepped lancets lighting the gallery stair, embellished with shafts and foliage capitals. The right-hand tower is brought forward in three stages with shallow set-back buttresses to the second and third stages above an offset. The lower stage contains double boarded doors with prominent false strap hinges within an arch incorporating a foliage frieze, surmounted by a mullioned overlight with shafts and foliage capitals. The right side wall features a cusped roundel, while the second stage has a single lancet below a roundel in the tympanum. The third stage contains a taller lancet with louvres. Above rises a parapet with blind arcade and polygonal panelled turrets over the buttresses, surmounted by an ashlar spire with a single tier of lucarnes.

The forecourt is enclosed by gates on both left and right sides, each with polygonal stone piers and pyramid caps. The gates are pairs of double iron doors featuring dog bars and scrollwork below spike finials. Between them runs a stepped dwarf wall of coursed rock-faced stone with iron railings of similar detail to the gates. The return wall on the right, abutting the tower, incorporates comparable detailing.

The right side wall is constructed of cyclopean granite, divided into five buttressed bays, each containing a pair of lancets. Behind the main chapel stands a two-storey vestry in materials matching the main chapel, with five windows facing Chapel Street. These are two-pane sashes under segmental heads, except for a pair of sashes to the lower left of the doorway. The doorway itself, positioned left of centre, has double boarded doors.

The main doorways lead to porches opening into the vestibule, which features two pointed lights to the main chapel with coloured glass. The porches and vestibule have decorative tile floors, half-lit panelled doors with coloured glass to the vestibule, and panelled doors incorporating coloured glass panels to the gallery stairs and main chapel.

The main chapel interior is spanned by a five-bay arched-brace roof with boarding behind, supported on a bracketed cornice with main trusses seated on corbels. Two cast iron columns with foliage capitals support a single-sided gallery, its front panelled and projecting on brackets. Each pair of windows contains a segmental pointed rere-arch. The organ recess is spanned by a tall pointed arch with clustered shafts bearing foliage capitals and a hood mould with foliage stops. A small traceried round window in the organ recess and the large window lighting the gallery feature non-figurative coloured glass. Flanking the organ recess are panelled vestry doors with coloured glass panels.

A large late nineteenth-century organ dominates the recess, housed within a panelled case of classical detail. It stands in a railed enclosure formed by twisted wrought iron uprights sprouting foliage-enriched scrollwork below a wooden hand rail, with square moulded wooden newels at the main corners. Within the enclosure stands a freestanding polygonal and panelled wooden pulpit on a boarded stem. The pews have panelled ends incorporating painted numbers in quatrefoil panels. A white marble memorial tablet in the side wall commemorates members lost in the First World War.