Crichton Castle comprises four contiguous buildings arranged around an inner courtyard. The 14th century tower lies at the east of the castle, and has a vaulted basement with timber entresol, and a vaulted hall above, although the south-west corner of the tower has collapsed. To the west of this tower was a barmkin. William Crichton extended the castle in the early 15th century, building a second tower to the south, forming a strong L-plan, with the gate between the two towers. The south tower was entered by a door in the centre, with vaulted cellars either side. Two halls occupied the first and second floors. In the later 15th century a west block was added, with a six-storey tower at the south-west, containing several bedrooms with garderobes. A stair in the south block gave access to these rooms. The north range was added at this time, closing the courtyard, but this section was heavily rebuilt in the following century.