The monument comprises Crookston Castle of medieval date, visible as an upstanding tower, enclosed by the earthworks of the first castle, along with an area surrounding the earthworks. The monument is in the care of the Scottish Ministers and was first scheduled in 1920. It is being rescheduled to extend protection to cover the whole of the archaeologically sensitive area.
The castle occupies the western part of a hilltop, dominating the confluence of the Levern Water and the White Cart, at between 20-30m OD.
Crookston exhibits at least two distinctive and major phases of medieval castle building, part of which may encompass the outworks of an earlier fort.
The primary work comprises the massive encircling bank and ditch of Robert Croc's earth-and-timber castle, built in around 1180. The ditch is up to 3.5m deep and forms an irregular hexagon shape in plan, with an entrance gap on its western side. Recent geophysical survey has revealed the remains of a circular structure and possible ditches some 40m E of the moat. These features may indicate the presence of a pre-existing ancient fort on this site, the western half of which may have been re-utilised by Robert Croc when he constructed his ringwork castle.
The secondary work comprises the impressive stone castle probably built by Sir John Stewart of Darnley at the beginning of the 15th century. The arrangement of this castle is unique in Scotland and comprises a high central oblong block (measuring c.19m E-W by 12m N-S) with four square corner towers. The two western towers and much of the western part of the main block have now gone, and the NE tower alone survives intact. This tower exhibits careful unitary planning. The public rooms were located within the central block, while the projecting corner towers contained the other accommodation expected in a conventional towerhouse: stores, kitchen, prison, bed chambers, and servants quarters. This castle was besieged by James IV's forces during a rebellion in 1489, although the defenders surrendered without a shot being fired. Labourers were brought from Paisley to partially demolish the castle, and although some of this damage was subsequently repaired, the western towers were never rebuilt. Crookston continued to be used as a residence until the end of the 16th century.
The area proposed for scheduling comprises the remains described and an area around them within which related material may be expected to be found. It is sub-rectangular in shape, with maximum dimensions of 225m ENE-WSW by 135m NNE-SSW, as marked in red on the accompanying map extract.