The monument consists of the remains of Auchinleck Castle, which possibly dates to the thirteenth century.
This early medieval tower occupies a strong position on the E bank of the Lugar Water. It is built on top of a sandstone rock stack which drops vertically to the river. The crumbling structure is made of local red sandstone rubble. Only the lower courses of the walls survive. These are over 1m thick and about 4m high in places. Part of the interior has been quarried out of the solid rock and the walling has augmented the natural defences provided by the rock pillar. The castle is approached by a rock-cut path which ascends to the summit from the S side of the steep rock face.
The area to be scheduled is irregular and measures a maximum of 70m NW-SE by 35m NE-SW to be centred on the castle, as shown in red on the accompanying map.
The monument is of national importance because it is a fortified site dating from the early medieval period; consequently it is of considerable historic value. It provides evidence, and has the potential to provide further evidence through analysis and excavation, for defensive architecture, domestic occupation and material culture during the period of its use.
RCAHMS records the site as NS42SE 2.