a German casemate at Omaha Beach, (Normandy) on Flickr.
Via Flickr:
Others (Devic) think that it comes from the Arabic word kasaba, transliterated to kasbah, the word that originated the Spanish word for fortress: alcazaba.[2] Menagio theorised it came from the Greek word for pit, khasma, the plural of which is khasmata.[2] Hensleigh Wedgwood thought that it came from the Spanish casa and matar, making a casemate a house in which killing happens. Others take matto in its archaic Italian meaning of dark, equivalent to the English matt, as in opaque, making a casamatta a dark house. Casematte were also used as military prisons, making use of their lack of light to add to the punishment. This explanation seems to be the most agreed upon.