Here you are, Lionello. I’ve included senses and earliest cites for each.
Forms: 15–16 casamat(t, casamate, (15 cassamate, 16 casemat, cazimate), 15– casemate.
Etymology: The actual form is < French casemate (in 16th cent. also chasmate , casmate , -matte ); the earlier forms were < Spanish casamata , Italian casamatta . Of these the first element is apparently Spanish casa and Italian casa house, but the second is uncertain. Diez mentions Italian matta in dialect sense ‘pseudo-’, also Sicilian matta dark. Wedgwood, comparing the English equivalent ‘slaughter-house’, suggests Spanish matar ‘to kill, slaughter’, but it is difficult on this theory to account for the form of the word.
a. Fortification. A vaulted chamber built in the thickness of the ramparts of a fortress, with embrasures for the defence of the place; ‘a bomb-proof vault, generally under the ramparts of a fortress, used as a barrack, or a battery, or for both purposes’ (Stocqueler 1853).
†b. An embrasure (obs.).The original sense is thus given by Barret Theor. Warres (1598) Gloss.: ‘Casamatta, a Spanish word, doth signifie a slaughter-house, and is a place built low vnder the wall or bulwarke, not arriuing vnto the height of the ditch, seruing to scowre the ditch, annoying the enemy when he entreth into the ditch to skale the wall.’ The Spanish and Italian is explained in the same words by Percivall and Florio; the latter adds as an English equivalent canonrie, i.e. cannonery n., loop-hole, embrasure.
1575 G. Gascoigne Noble Arte Venerie sig. A.iiij, Plotformes, Loopes and Casamats, deuisde by warlike men.
†c. fig. ? Batteries.
1635 T. Heywood Hierarchie Blessed Angells vii. 441 Of Thunder, Tempest, Meteors, Lightning, Snow, Chasemates, Trajections, of Haile, Raine.
d. Naut. An armoured enclosure for guns in a warship.
1888 Engineering 17 Feb. 159/2 Italian Ironclad ‘Italia’.‥ The barbettes are contained in an armoured casemate, which is supported by the unarmoured structure of the ship.
2. Archit. ‘A hollow moulding, such as the cavetto’ (Gwilt); = casement n. 1.
1611 R. Cotgrave Dict. French & Eng. Tongues, Nasselle,‥a hollow in a piller, etc., called, a Casemate.