Cairns Castle is though to have been built around 1440, and comprises of a rectangular main tower attached to which was a smaller square wing.
The castle stands on a raised mound jutting out into the Harperrig Reservoir where it is fed by the Water of Leith.
The main tower is three storeys in height, and measures around 8.3 metres by around 7.2 metres, with walls varying from 1.4m to 1.8m in thickness. The ground floor is vaulted and contained the kitchen. An arched doorway gives access to this level.
The secondary tower, the majority of which was removed around 1870, was attached to the south-east corner of the main tower, and measured around 5.5 metres square. It also had a vaulted basement, and there was an intramural spiral staircase close to the re-entrant angle to the north-east. The main entrance to the castle seems to have been a rectangular doorway on the east wall of the first floor.
While the main tower is a substantial ruin, only a lower section of the smaller wing stands. Many of the remaining details, such as the main entrance on the first floor, are now obscured by thick ivy.
Cairns Castle was the seat of the Crichtons of Cairns. In 1418 a Stephan de Crechtoun is described as “de Carnis”, and it is thought that his son, George de Crychtoun, built Cairns Castle around 1440. George would later be created Earl of Caithness in 1452 by James II. The Cairns estate may have later passed to the Tennant family.