The monument comprises the roofless shell of a late medieval tower and a set of surrounding earthworks, which may be of rather earlier date.
The monument lies at around 125m OD, on a prominent knoll at the mouth of Glen Eagles. The tower measures approximately 13.5m E-W by 7.5m within walls 2.3m thick. The remains of an entrance door can be seen in the SE corner and there is evidence for spiral staircases in the SE and NW corners. Little survives of the E wall but the N, S and W walls survive up to 6m high with windows at ground and first floor level, and evidence of a possible vaulted ceiling over the ground floor level.
At least three of the ground level windows feature inverted keyhole gunloops, a style popular in the late 15th century. A feature in the SW corner may represent an oven and the remains of a drain or garderobe chute can be seen emerging from below the N wall. The footings of a later rectilinear building or enclosure can also be traced inside the shell of the tower, built against the N wall.
The tower is surrounded by a clearly defined earth and stone bank that follows the summit of the knoll. This bank measures up to 7m in width at its base, standing about 1.3m high internally and 2.7m externally. This bank probably represents the remains of a barmkin wall, although it could also incorporate the remains of a small Iron Age earthwork, for which the location would have been suitable.
Two large mounds within the barmkin on the E side of the tower may represent debris from consolidation works carried out in the 1920's. These works are commemorated on a plaque, situated in the central ground level window of the N wall, which reads "Gleneagles Castle. Repaired by Sir General Aylmer Haldane under the direction of HM Office of Works 1927-28."
The area proposed for scheduling comprises the remains described and an area around them within which related material may be expected to survive. It is irregular in shape and measures a maximum of about 95m E-W by 100m N-S, as marked in red on the accompanying map extract.