The cave is located 26 meters below the level of the IV. courtyard. It was originally accessible only via the throat of a well in the courtyard, which was probably dug in the place of the original narrow channel that brought water to the surface. The cave consists of a circular hall with a diameter of 22m and a height of 6m. The well was cut down to a level 4m below the bottom of the hall. The lower part of the cave is flooded with water, the deepest pool is up to 9 meters deep. The air temperature in the cave is all around 8 °C all year long.

Bojnice castle cave is one of the most remarkable caves in Slovakia. It was not formed by erosive activity like the stalactite caves in karst areas, on the contrary - the cave literally grew in its place. The age of the travertine is estimated at 297 000 to 19 000 years. The genesis of the cave is evidenced by the so-called grape-like or lumpy formations around its entire perimeter, which were formed in the thermal water below its surface, which distinguishes them from the typical features of stalactite caves, formed by the erosive action of water - stalagmites, stalactites, stalagnates.