Casemates are not really underground, they are in general built as the lower levels or the basements of a fortress. Nevertheless casemates are generally considered to be subterranea. We guess its because most casemates are rather old (several centuries at least) and during the centuries speleothems grew on ceiling and walls from the lime in the cement. Or its the low temperature and the high humidity which is very similar to a natural cave.
Casemates feature high vaulted chambers, extremely thick walls, often with armoured doors and very often with openings for guns. We include on showcaves.com only the most impressive casemates relevant for tourists, as this topic would generally require a whole website on its own.