The monument consists of a roofless castle, built on a slight headland, together with an area corresponding to part of its associated barmkin.
The castle, first recorded in 1563, probably dates from the second half of the 16th century, and can be related stylistically to a group of castles in Ayrshire and Argyll, such as Gylen Castle. The Z-plan tower house consists of 4 storeys (the lowest vaulted) plus attic, both jambs being circular or near-circular on plan. The main block is very narrow in relation to its height, measuring 8.4m NW-SE by 7.2m SW-NE over walls 1m in thickness, and the effect of loftiness seems to have been deliberately enhanced by tall chimneystacks. The N angle of the castle has collapsed. The jamb at the W angle contains the remains of a newel stair giving access to the first and second floors; at this level a stair turret is corbelled out of the NW wall to give access to the upper floors. A modern buttress supports the W angle of the W jamb, and on the inner side of the jamb are traces suggesting that a former entrance has been blocked. Both the corbelled base of the stair turret and the dormer window in the SW wall are heavily ornamented with billet moulding.
The sides of the promontory are revetted, but there are no other obvious traces of the castle to the NW of the tower, although it is likely that there was a barmkin, or outer enclosure, on this side.
The castle is dramatically sited, springing from the cliff top to SW, SE and NE, the effect apparently deliberately accentuated by the massing of the design, which is very tall for its ground plan.
Keiss Castle is recorded as being in the ownership of the Sinclair Earls of Caithness in 1623, although by 1681 it was held by a cadet branch of the family. It is described as being ruinous in 1700, but this may have been exaggeration since it was in good repair in 1726, sited beside "a convenient house lately built." The reference of 1563 to a castle at Keiss may not refer to the present structure, opening the possibility of earlier occupation on the site.
The area to be scheduled is irregular in shape and measures a maximum of 42m E-W by a maximum of 18m N-S, as indicated in red on the enclosed map. It is defined by the inner side of the modern fence to the NW and by the top edge of the cliff on the other sides. It includes the castle, the revetted sides of the promontory, and an area of ground between the castle and the modern fence where the house recorded in 1726 will have stood, presumably preceded by a barmkin.