The most significant buildings were established in the later 15th century through into the 16th century when David's Tower (also known as Davey's Tower) was built along with other substantial accommodation areas. The tower is the largest by volume of all medieval Scottish towers measuring 19m by 13.5m and 22m in height and was started by Bishop David Stewart (1462–76) and completed by Bishop William Tulloch (1477–82). It has 6 storeys above ground and had a garret on the top. Below ground is a vaulted basement which is approximately 1.5m below the level of the courtyard; it contains a circular dungeon, 5.3m in diameter, lit only by a narrow loop looking to the west and has a slightly domed vaulted roof. The ground floor contained the hall which is 12.8m by 6.7m and lit by large windows with built-in stone seats. At the NE corner was a spiral staircase which led to the upper floors. Each of these floors were similar in design with a single large room and smaller chambers leading off it. The tower is plain and practically lacking in architectural features; the external walls were originally rendered while the internal walls were plastered.