Old Castle Lachlan, which is probably of 15th century date is of a peculiar form.
Externally,its appearance is of a great keep, 70' N-S by 54' transversely, standing 43' high; but on going inside, it is found to consist of two tenements, on the E and W sides of a narrow open court, 12'6" wide by 34'3" long. These two blocks are connected at the N end by a small building containing a room 8' by 7' and a small wheel-staircase. The castle was occupied by the MacLachlans till 1746, when slighted by government forces; probably members of the Argyll Militia who had occupied it since the Chief left for Edinburgh to support the Stuarts in 1745. It was never again occupied.