Kilmartin Castle is a small formerly ruined 16th century Z-plan tower house. It consists of a main block of three storeys, with projecting round towers at opposite corners. A small stair-tower stands against one side of the main block. The walls are pierced by shot-holes, and the windows still have iron yetts.
The entrance leads to the vaulted basement which contains three chambers: one, the kitchen, linked by a passage. A turnpike stair leads to all floors in one of the round towers. The hall, with a large fireplace, was on the first floor of the main block and had an adjoining private chamber. There were more private rooms above the hall.
The castle was the residence of John Carswell, who moved to Carnasserie Castle when made Protestant Bishop of the Isles. It later became a property of the Campbells and was restored by the late Tom and Olive Clarke.