The date present on one of the quoins on the west side of the stair tower at a height of around 3m. indicates "1375"; this is inconsistent with such an L-shaped tower, but it is suggested (J. Zeune, "The Last Scottish Castles", VML, 1992) that mediaeval masons often carved a "5" upside-down so that it appeared as a "3", which would then indicate a date of 1575 (or 1573). Such a date for the addition of the stair tower would be consistent.

Structural evidence suggests that the stair tower was an addition to the earlier rectangular tower, giving rise to the present L-form. The presence of an apparent filled door at a height of around 2m on the east side of the rectangular part, and the better state of the stair tower, provide structural support for this. A door at first floor level is a defensive feature characteristic of the 15th rather than 16th century. Carsluith Castle, close by, is considered to date from the early 16th century and appears only ever to have had ground level access . The fact that the stair tower is considered to date from late in the 16th century or early in the 17th century is however at odds with the story of John Knox having hidden in the arched room at the head of the staircase (the present cap house?) around 1566; the first written record of this appears to have been made over two and a half centuries later, based on an account given by the great grandson of a man who recalled John Knox's signature being present on the wall of the room at the top of the stair tower.