Hillslap Tower is a 16th century L-plan tower house of four storeys. It consists of a main block and a stair-wing, which rises a storey higher. A stair turret projects from above first floor level in the re-entrant angle. The walls are pierced by gunloops and the tower is dated 1585. The tower has a courtyard with a range of buildings..
The entrance leads to the main stair, which climbs to the large hall on the first floor of the main block, the other floors reached by the stair turret. The basement is vaulted.
Hillslap was a property of the Cairncross family. Walter Scott used it for his novel "The Monastery", calling it 'Glendearg'. It was re-roofed and restored in the 1980s.