The Cave comprises cylindrical lower and bell-shaped upper parts totalling 17ft (5.2m) diameter and 25½ft (7.7m) height. Between the two parts is a frieze marked in a diamond fashion, while the lower part is decorated with low relief carvings of medieval appearance, some of which were originally coloured. Some believe the Cave's shape is modelled on the Church of the Holy Sepulchre in Jerusalem.
What may be beam slots can be seen under the entrance and in the diagonally opposite part of the wall, indicating there may once have been a wooden part-floor above the frieze. An investigation of the floor in 1976 found a post slot, which might have supported the floor above. It has been suggested that the structure was a star shaped platform supported on four posts held in position by struts wedged between the platform and the cave wall. Such a structure would have given access to the line of large niches at a level about 13ft (4m) above the Cave floor. These niches were clearly intended for storage but of what there can be no certainty.
Around the base of the cave there is a raised octagonal step, badly worn in places at the time before the railings which follow the line of the octagon, were fitted. The step, which is about 8in (20cm) deep and too low for sitting, was probably used for kneeling while contemplating the carvings.
The carvings in the Cave include four saints. Below the original entrance is St. Christopher, patron saint of travellers, with the child Jesus on his shoulder and staff in hand. Moving to the left high up on the west part of the wall is St. Katherine. Farther to the left is St. Lawrence who was martyred on a gridiron. Next is the figure with drawn sword which might be St. Michael or possibly St. George, patron saint of England who in symbolic terms saved the lady (the church) from the dragon (the devil or the oppressor). St George's sword points to what might be the twelve apostles with Judas the small figure at the back towards the right-hand end of the row.
The large panel on the left of St. Christopher represents the Holy Sepulchre having a damaged figure of Christ awaiting the resurrection above the large niche on the left. Mary Magdalene, or an angel on the right-hand side sits on the stone rolled away from the entrance. The dove and the hand above may represent the Holy Spirit. The niche below probably held a lamp.
The long row of figures below includes both men and women and although none can be identified those marked with crosses are possibly saints and those with hearts may be martyrs. The two small figures below St. Katherine may be (although this is by no means certain) Richard I (Lion Heart) and his Queen Berengaria whose crown is shown floating above her head as she was never actually crowned Queen. The long vertical cross on their left is a branched or floriate cross and represents the high altar. Although currently identified as a cross there is a possibility that it represents a sword.
Beneath St. Lawrence is a figure with upraised arms that has been variously identified as King William of Scotland and King David. Interestingly, a similar figure appears at the head of a 13th century illuminated manuscript of Psalm 69, now in Trinity College Library, Cambridge, where King David is shown rising from the water. Psalm 69 is a plea for help and a lament about being misunderstood.
Elsewhere in the Cave are Calvary scenes with Mary and John and a group believed to show the Holy Family but uncertainty surrounds the remaining figures in the Cave. There are several large niches among the carvings, which may have been for relics or lights.
The whole surface of the lower part of the Cave is covered with names cut by visitors, a result of the uncontrolled access to the Cave before the time when Royston Town Council took over the Cave and installed the railings to prevent visitors touching the soft carvings.