The college buildings at Carmarthen were built as South Wales and Monmouthshire Training College, for teacher-training, in 1847-8 by Henry Clutton for the National Society for the Education of Children, the principal Anglican educational body. The original building was in Puginian Gothic style, an early example in Wales, and comprised the principal's house to the left, the main range with dormitories for 60 above the dining-hall and library, and a small chapel at the east end. The front elevations vary slightly from a 1848 lithograph, and a rear wing was added in 1860. Additions in 1906-12 included an examination hall and a very large Gothic chapel in 1931-2, at right angles to the original chapel. The college was renamed Trinity College in 1931.

Built of rubble stone with tooled sandstone quoins and Caen limestone dressings, steep slate roofs; the college is two storeys and attic, with the main front to the south: a single range with a ridge octagonal lantern, flanked by coped gabled projections. The lantern was originally open, now closed in copper-sheet.

Internally, there is a tiled hall with a segmental pointed door into the altered library, and plain stone cantilevered stairs. The old chapel of has a fine painted roof with deep arch-braced scissor trusses on the corbels. The new chapel is large with a nave and chancel. It has a hammerbeam roof of 7 main trusses and 6 intermediate trusses. One side window has a 1996 stained glass of the Black Book of Carmarthen, by Gareth Morgan.

Reference: Cadw listed buildings database.