Royston Cave is a small artificial cave in Royston in Hertfordshire, England UK (Wikipedia page  refers). It is located beneath the crossroads formed by Ermine Street and the Icknield Way. It has recently been speculated that it was used by the Knights Templar, who founded nearby Baldock, but this is unlikely, despite its enormous popular appeal. It is more likely that it originated as a cell for anchorites from the nearby Augustinian Priory, although a recent suggestion is that it was the town's earliest prison, perhaps dating from the late fifteenth century. It is open to the public in the summer months on Saturday, Sunday and Bank Holiday afternoons between Easter and October.
Royston Cave is a circular, bell-shaped chamber 8 metres (26 feet) high and 5 metres (17 feet) in diameter with a circumferential octagonal podium. The origin of this chamber is unknown. This cave is unique in Britain - if not the world - for its numerous medieval carvings on the walls. Some of the figures are thought to be those of St. Catherine of Alexandria, St. Lawrence and St. Christopher.
More information, and links, on the Wikipedia page