Only those parts of the castle generally accessible to visitors are recorded in this description. Although not described here much of the furniture and many of the paintings (including family portraits) are also original to the house. Similarly, it should be noted that in the interests of brevity and clarity, not all significant architectural features are itemised in the following description.
Entrance gallery: one of the last parts of the castle to be built, this narrow cloister-like passage was added to the main block to heighten the sensation of entering the vast Grand Hall, which is made only partly visible by the deliberate offsetting of the intervening doorways; bronze lamp standards with wolf-heads on stone bases. Grand Hall: entering the columned aisle of this huge space, the visitor stands at a cross-roads between the 3 principal areas of the castle's plan; to the left the passage leads up to the family's private apartments on the 4 floors of the keep, to the right the door at the end leads to the extensive service quarters while ahead lies the sequence of state rooms used for entertaining guests and displayed to the public ever since the castle was built. The hall itself resembles in form, style and scale the transept of a great Norman cathedral, the great clustered columns extending upwards to a "triforium" formed on 2 sides of extraordinary compound arches; stained glass with signs of the zodiac and months of the year as in a book of hours by Thomas Willement (completed 1835). Library: has very much the atmosphere of a gentlemen’s London club with walls, columned arches and ceilings covered in the most lavish ornamentation; superb architectural bookcases and panelled walls are of oak but the arches are plaster grained to match; ornamental bosses and other devices to the rich plaster ceiling refer to the ancestry of the Dawkins and Pennant families, as do the stained glass lunettes above the windows, possibly by David Evans of Shrewsbury; 4 chimneypieces of polished Anglesey "marble", one with a frieze of fantastical carved mummers in the capitals. Drawing room (great hall of the late C18 house and its medieval predecessor): again in a neo-Norman style but the decoration is lighter and the columns more slender, the spirit of the room reflected in the 2000 delicate Maltese gilt crosses to the vaulted ceiling. Ebony room: so called on account of its furniture and "ebonised" chimneypiece and plasterwork, has at its entrance a spiral staircase from the medieval house. Grand Staircase hall: in many ways the greatest architectural achievement at Penrhyn, taking 10 years to complete, the carving in 2 contrasting stones of the highest quality; repeating abstract decorative motifs contrast with the infinitely inventive figurative carving in the newels and capitals; to the top the intricate plaster panels of the domed lantern are formed in exceptionally high relief and display both Norse and Celtic influences. Next to the grand stair is the secondary stair, itself a magnificent structure in grey sandstone with lantern, built immediately next to the grand stair so that family or guests should not meet staff on the same staircase. Reached from the columned aisle of the grand hall are the 2 remaining principal ground-floor rooms, the dining room and the breakfast room, among the last parts of the castle to be completed and clearly intended to be picture galleries as much as dining areas, the stencilled treatment of the walls in the dining room allowing both the provision of an appropriately elaborate "Norman" scheme and a large flat surface for the hanging of paintings; black marble fireplace carved by Richard Westmacott and extremely ornate ceiling with leaf bosses encircled by bands of figurative mouldings derived from the Romanesque church of Kilpeck, Herefordshire. Breakfast room has cambered beam ceiling with oak-grained finish.
Grand hall gallery: at the top of the grand staircase is vaulted and continues around the grand hall below to link with the passage to the keep, which at this level (as on the other floors) contains a suite of rooms comprising a sitting room, dressing room, bedroom and small ante-chamber, the room containing the famous slate bed also with a red Mona marble chimneypiece, one of the most spectacular in the castle. Returning to the grand hall gallery and continuing straight on rather than returning to the grand staircase the Lower India room is reached to the right: this contains an Anglesey limestone chimneypiece painted to match the ground colour of the room's Chinese wallpaper. Coming out of this room, the chapel corridor leads to the chapel gallery (used by the family) and the chapel proper below (used by staff), the latter with encaustic tiles probably reused from the old medieval chapel; stained and painted glass by David Evans (c1833).
The domestic quarters of the castle are reached along the passage from the breakfast room, which turns at right-angles to the right at the foot of the secondary staircase, the most important areas being the butler's pantry, steward's office, servants' hall, housekeeper's room, still room, housekeeper's store and housemaids' tower, while the kitchen (with its cast-iron range flanked by large and hygienic vertical slabs of Penrhyn slate) is housed on the lower ground floor. From this kitchen court, which also includes a coal store, oil vaults, brushing room, lamp room, pastry room, larder, scullery and laundry are reached the outer court with its soup kitchen, brewhouse and 2-storey ice tower and the much larger stables court which, along with the stables themselves containing their extensive slate-partitioned stalls and loose boxes, incorporates the coach house, covered ride, smithy tower, dung tower with gardeners' messroom above and footmen's tower.