Tudor Gothic mansion of 2 stories and attics, of ashlar stone under renewed slate roofs. The house is characterised by its asymmetry (except for the W garden front) and its rich Gothic detail. Irregular gabled bays surround a courtyard and a prominent octagonal tower. Detail includes turrets, crenellated parapets above a continuous string course with decorative bosses, finials, bay windows, oriel windows, a plinth, ornate chimney shafts in groupings of up to 6, and tablets bearing heraldic emblems. Tudor-style windows under hoodmoulds, with central transom and a varied number of lancet-headed lights, mostly containing renewed horned sash windows.
The entrance front is to the N, an advanced gabled bay R of centre containing the porch. Further L is a gateway leading into a service courtyard. The W end of the house forms an impressive garden front overlooking the broadwalk which leads down to the orangery and abbey buildings. A further garden front is to the S overlooking the deer park.
The gabled N entrance bay is 2-storey, the lower storey projecting slightly. Four-centred arched doorway with several orders of mouldings under a hoodmould. Double wooden doors, ribbed and studded with 2 tiers of lancets above mid rail. A panel above includes foiled tracery in the spandrels bearing Mansel and Talbot heraldic emblems. The apex of the arch rises to a shield bearing a lion motif. In the 2nd storey is a 4-light Perpendicular window with pointed head. Flanking octagonal turrets, highly decorated, rising to circular shafts with lattice and quatrefoil tracery under domed caps with finials. Four-light Tudor window to E side of entrance bay. To its R is the gable end of the W front, with pronounced stepped chimney breast. At each stage is a panel with shields, tracery and foliage. Four clustered shafts rise from the stack with varied decoration. Slender octagonal angle turrets rise to circular shafts with lattice and quatrefoil tracery. To the L of the entrance bay are 3 bays, the central bay advanced, beyond which is a large advanced gabled bay, the E side of which forms the boundary of the service courtyard. The 3 bays have stepped gables incorporating half dormers, 2-light to L and centre and 3-light to R. Tablets with shields in gables. The string course forms a continuous hoodmould. The wider advanced central bay has a 4-light window below the attic half dormer and a 6-light bay window with hipped roof to the ground floor. Large octagonal stair turret to R angle bearing narrow lights, that to attic trefoiled with triangular hoodmould, and with battlemented parapets rising from corbel table. The R bay is 2-window under the dormer, with 2-, 3- and 4-light windows. The L bay is 1-window, each of 2-lights. The wide advanced gabled bay to the far L is dominated by an oriel canted bay window at 1st floor level, with crenellated parapets and a hipped roof. Flanking at ground floor level are 2-light windows.
Two-storey symmetrical W front with crenellated parapets above a string course with head and foliate bosses. The elevation is well lit, the Tudor windows having cinquefoil-headed lancets. Full-height canted bay to centre flanked by semi-octagonal bays. The centre bay has 3-light windows and a gable bearing a shield rising above flat-headed parapets. The semi-octagonal bays are narrower to the upper storey and have a light to each face. These bays rise above the parapets with a shield to each face, narrowing to octagonal pinnacles with traceried openings to each face, under ogee domes. The flanking walls each have a full-height canted bay in the centre containing a 3-light window. Two-light windows flanking, to upper storey only. Slender octagonal angle turrets with traceried circular shafts capped with domes.
Symmetrically placed and rising above the central canted bay of the W front is the octagonal tower. Two stages rise above the roof line with slender octagonal turrets at each angle. Three-light windows to lower stage with quarries, above which is a wide frieze with lions and shields in relief. Windows of 4 narrower lights above, surmounted by crenellated parapets with openwork tracery. The circular shafts of the angle turrets, with lattice and quatrefoil decoration, rise high above the parapets. Octagonal stair turret on E side, plain with occasional slits. The upper level, above the tower parapets, is faced with blind trefoil-headed lancets. (The parapets of the stair turret have been truncated.)
The long S front of Margam Castle is highly asymmetrical. There are 6 bays including 3 advanced gabled bays, and towers and turrets. Detail includes string courses at each level; bosses with monks heads, Tudor roses, lions, shields and beasts; gable finials with barley twist decoration, crenellated parapets and windows in the same style as the W front. To the L is the wide advanced gable end of the W front, with slender angle turrets. In its centre is a 2-storey canted bay with openwork traceried parapets, 3-light windows and side lights. To the R is a 3-window bay containing a central full-height canted bay with 3-light window to each storey, flanked by 2-light windows to the ground floor and single-light windows above. On its R is a square stair turret, slightly advanced, with a single light window to each storey, that below with a shield beneath a raised hoodmould. At mid level is a small oriel window. Above parapet level, the turret has a blind light and a quatrefoil frieze, and is surmounted by a cluster of 6 chimney shafts. To the R of the stair turret, the upper windows are at a lower level and there is an attic storey. Immediately to the R is a large advanced gabled bay incorporating a garden entrance to the morning room under a 4-centred arched head with Y-tracery. Glazed door to each side of the central mullion with Gothic tracery. Square hoodmould with shields in the spandrels of the arch. Immediately above is a small decorated limestone block known as the Palladium stone. The emblem is hard to decipher, but is said to confer prosperity to the building. In the storey above are 2 single lights with cinquefoiled heads containing 4-over-4 pane sashes under triangular hoodmoulds. Oriel canted bay window of 2-lights to attic storey with crenellated parapets and hipped roof. Single light window to its L, and 2-light window in gable apex. Projecting from the R angle of the advanced gabled bay is an ornate octagonal turret with single light windows to the lower storey. The storeys above become narrower with blind, mainly ogee-headed lancets. At parapet level is an ornate frieze surmounted by a polygonal shaft with openwork panels of quatrefoils under a domed cap. To the R of the turret is a bay on the original alignment. A panelled half-lit door to the L (possibly a later insertion) leads into the study with 2 windows to the R, all under a single hoodmould. Two-light windows to 1st and attic stories; the latter is a half-dormer with an attic frieze including a shield. To the R is a 3rd projecting gabled bay with slender angle turrets and a shield in the gable. In the centre is a 2-storey bay projecting on a corbel table with trefoiled decoration. Decorated panels at ground floor level, and a 4-light window to 1st floor with hipped roof. On the E side of this projecting bay is a 3-light window to the ground floor and 2-light window to the attic. Set back to the R and on the original alignment is a square stair tower. Two-light window to ground floor and single lights to 1st floor and attic. The tower rises above and is slightly set back with chamfered angles. Square panel to front bearing a cinquefoil and a shield, above which are tall crenellated parapets. A small octagonal turret rises to the rear and provides access to parapet level of the tower. Recessed slightly to the R is the gable end of the kitchen range, with large projecting decorated chimney breast and angle turret to R. The stack has a corbel table at parapet level and then narrows. Three shafts rise from the stack, but the central one is a bell cupola. A boundary wall to the service areas runs E from the decorated angle turret.
The E side of Margam Castle is irregular and is joined by service buildings and courtyards. However, it includes the main kitchen and other service rooms which project to the L. The L side of the kitchen is dominated by a large bay window with a hipped roof. A shield in relief rises from the centre of its eaves. The window has a central chamfered mullion flanked by 3 lights with 2 transoms and contains diagonal quarries. Boarded door to R under chamfered 4-centred arch, within a small enclosure, formerly the Cook's room with the remains of a 3-light window. Two-light window above with lancet heads, and 2 to attic storey. Eaves stack with 2 octagonal shafts L of centre. The N side of the kitchen is gabled with a central 4-light window to the ground floor and 2-light windows to the attic flanking a stack projecting on corbels. Two adjoining gabled bays to the W, also with irregular 2-light windows to 1st floor and attic. The central gabled bay is butted by a long single storey range which runs to the N and forms a corridor to a courtyard behind, which was later converted to a billiard room. It has a porch to R of centre with a cross-window to the front under a stepped gable with blind window containing Y-tracery. Doorway into L side under a chamfered lancet head. Cross-windows flanking porch, and similar 3-light window to L. Small bay in L angle with hipped roof and cross-window. To the R, this corridor continues as a lean-to against a gabled bay relating to the N front. It has a hipped roof, beyond which is a square flat-roofed structure with lancet-headed doorway to L side which forms a buttress to the main gateway to the service courtyard.
The courtyard which contained the billiard room has lean-tos to N and S. That to S has large Tudor arched windows to ground floor lighting service areas. Otherwise, mainly 2-light windows containing sashes in square stone surrounds, with some cross-windows. Late C20 doors to W and N. The E wall is rendered and contains a fireplace.