Ruins of abbey church. The plan is not wholly conventional, though it was to the Cistercian cruciform model, square-ended with three chapels on the east side of each transept. The nave was aisled with the arcades raised on screen walls some 5-7ft high, a feature not found elsewhere in England or Wales but known in Ireland. Evidence for a crossing tower is uncertain: there is a mural stair in the N transept, a great bell was bought in 1255 and the fire of 1284 started in the belfry, which presumably was over the crossing. The presbytery was extended by one bay, possibly when building or soon after, and there is evidence for the floor being raised in the C14 when it was paved with encaustic tiles. A further raising of the sanctuary probably occurred in the C15 when the altar was brought forward and two small chapels were inserted against the east wall.
The extensive carved fragments found in 1887 are enough to confirm stone vaulting of the presbytery and the transept chapels with ribbed vaults. The presbytery vault ribs seem to be the source for the stones with a double band of big roundels. Some have been found built into the parish church, Great Abbey Farmhouse and Trawsgoed mansion. Capital details were of a transitional kind, more stiff-leaf than scallop, and with a certain amount of Celtic interlace. The nave arches were pointed and the arch mouldings of the crossing showed a round moulding to the earlier E arch, while the other three were beaded square. Three suggested forms of the nave piers: a thick quatrefoil cluster with shafts in the clefts; a round pier with four attached shafts and a cross with rebated angles and shafting at all the corners. The crossing, and in a minor form the transept chapel piers, were square with half-round responds and rebated angle shafts.
Apart from the W walls the remnants are all low wall-bases with some moulded detail to the bases of crossing arches, transept chapel arches and the the W end of the arcades. The W door is a remarkable piece, illustrating the separation of the far west from the mainstream of the Romanesque. Five orders of shafts are carried right around the arch without capitals, but are interrupted regularly by ribbon-like bands terminated in a scroll, four across the uprights each side and five radiating around the arch, the centre band ended in a double scroll interlaced with leaves; there is also a scroll-ended hoodmould. The aisle window to the right is a pointed lancet. Analysis of stones found in the 1887 excavation suggested a very poor quality lime/mortar used with local stone. However there seem to have been dressings of four types: a millstone grit (apparently reused from an earlier building as there was moulding reversed in the main wall), Bath limestone for the dressings and quoins, some yellow Dundry stone and some purple Carbwdy stone from St Davids, which Williams thought was used in banding the arches. Much evidence of fire in the fragments. Remarkable survivors are the C14 tiles with which the chancel and the transept chapels were paved. Some are simply impressed with the decoration, some overlaid with white slip. The patterns are mostly geometrical, but there is a man witha mirror in C14 dress, the long liripipes dangling from his sleeves, a dragon, and a griffin.
Of the monastic buildings, a narrow sacristy opening into the church is integral with the S transept. The Chapter House beyond was initially large, some 54ft by 27ft, but was halved in size in the C14, an indication of reduced status, and possibly put to use as the Warming Room. The base of the C15 N cloister wall shows a five-bay range with five-light windows and a central reading alcove. The W range is almost all gone; this was probably the lay brothers' section and the S range is under the curtilage of Great Abbey Farmhouse.