Harperrig Reservoir Circular Walk.

Cairns Castle is a ruined keep, dating from the 15th century. It is located on the northern slope of the Pentland Hills, around 6.5 miles (10.5 km) south west of Balerno, at the south west end of Harperrig Reservoir, in West Lothian, Scotland.It may have also been known as Easter Cairns Castle, but this may refer to another castle in the area.

Harperrig Reservoir is a reservoir in West Lothian, Scotland, UK, to the west of the Pentland Hills, four miles south of Mid Calder.The Water of Leith flows through it, and nearby are Eliburn and Cobbinshaw Reservoirs.