The monument comprises the remains of a late medieval castle, which survives as substantial stone structures, earthworks and buried archaeology, together with an area enclosing the outer defences.
The castle is located on a promontory above the deeply wooded gorge of the Avon Water, bounded by steep cliffs to N and E. The main elements of the castle are set below the brow of the hill, below the direct sight-line of any besieging artillery.
Cadzow is rarely mentioned in historic sources, although the architectural evidence'for example, the 'dumb-bell' and 'wide-mouthed' gun loops'suggests that it was built sometime between 1500 and 1550. It is possible that Sir James Hamilton of Finnart began its construction when he was made guardian of the 2nd Earl of Hamilton in 1529, and the earl may have continued work on it himself in the 1540s. The Hamiltons were very much to the fore in the politics of state surrounding Mary Queen of Scots, focussed on Hamilton castle which is located only 2.3km to the N. Cadzow is likely to have been involved in the considerable military activity in the area at this time. The Queen took Hamilton and Craignethan in 1565. Hamilton was taken again in 1568, then in 1570, and finally in 1579 when cannon were employed from Edinburgh Castle. Hamilton castle was demolished at this time and, although no specific mention is made of Cadzow, it is likely to have met a similar fate.
The castle consists of three concentric parts:-
1. Outer ward:' this is a large ditched enclosure to the W and S of the core, roughly triangular in plan and over 300m wide along the line of the ditch. The S part has been heavily disturbed by the construction 19th-century roads. The entire enclosure is currently overgrown with trees and vegetation, almost impenetrable in parts.
2. Middle ward:' the middle ward is now chiefly visible to the W, while the S part is cut through by the roads mentioned above. Masonry structures have been observed to the S of the road, and recorded in the RCAHMS plan. The N range of the middle ward is the best-preserved structure, 32m long with a 9m wide basement, originally with a timber first floor. Both floors had rows of windows, the effect being reminiscent of a barracks.
3. Inner ward:' the inner ward is comprised mainly of an overgrown mound, masking ranges and/or a tower on the cliff-edge. The N façade, above the gorge has been the subject of current repair works following collapse in 1999. The two S corners have projecting cylindrical towers. A vertical shaft is visible at the NW corner, possibly a well. There is evidence of a timber bridge entry on the W side over the rock-cut ditch that divides these two wards. The principle domestic apartments are likely to have been within the inner ward. The large upstanding W-facing wall has a pronounced batter, which is most unusual.
Cadzow may not have been re-occupied following the siege of 1579. The 5th Duke of Hamilton built the hunting lodge of Chatelherault close by, to the NE, in 1732. From then on the rugged picturesque ruins of Cadzow were incorporated into the formal landscaping of the High Parks of Hamilton Palace, providing a gothic counterpoint to the classical symmetry of Chatelherault. Recent archaeological recordings as part of current consolidation works has revealed that the wall heads on the N side of the inner ward had been artificially raised, and that a thick clay capping was spread over the structures here to act as waterproofing. Elsewhere the castle exhibits strong evidence of masonry works to 'romanticise' the ruins in the 18th and 19th century.
This part of the Hamilton High Park was purchased by the Secretary of State for Scotland in 1978. Cadzow Castle remains in the ownership of the Scottish Ministers and is cared for by Historic Scotland.
The area proposed for scheduling comprises the remains described and an area around them within which related material may be expected to be found. The area proposed for scheduling is the same as that of the property in the ownership of the Scottish Ministers. The area is irregular in plan with maximum dimensions of 109m N-S by 186m E-W, as marked in red on the accompanying map. The modern fences are excluded from the scheduling, as is the upper 300mm surface of all modern roads and paths, to allow for their maintenance.