Translating a civil engineering document, I had a hard time with the word “casemate” (or more precisely, with what I sincerely hope is its Hebrew equivalent) - a word I’ve never before used in my life, in either language. The English dictionaries I see are more or less unanimous in deriving the word from the Italian casamatta (a crazy house? a dark house?) though the connections suggested are somewhat ambiguous. Moreover, the word means something quite different today from what it may have meant several hundred years ago. Wikipedia has a very interesting short article, and cites several sources which propose alternative etymologies. I’d be very interested to see what the OED says about it, if anybody will be kind enough....
The word is apparently sometimes confused with “casement”, which is a wholly different kind of structure.

Here you are, Lionello. I’ve included senses and earliest cites for each.

From the OED:

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.

1.

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.

The OED is not the reference of choice in this case, both definition and etymology having been published in 1888 (and written who knows how long before that).  The brand-new Concise Oxford defines it as “a small room in the thickness of the wall of a fortress, with embrasures from which guns and missiles can be fired”; the etymology derives casamatta “perh. from Gk khasma, khasmat- (see chasm).”

An Universal Military Dictionary, a Copious Explanation of the Technical Terms &c. Used in the Equipment, Machinery, Movements and Military Operations of an Army, published in 1779 by Captain George Smith, Inspector of the Royal Military Academy at Woolwich (and therefore as authoritative in its time as a book could imaginably be) defines casemate as:

in fortification, a vault, or arch of mason-work, in that part of the flank of a bastion which is next the curtain, made to defend the ditch, and the face of the opposite bastion. 

Thank you, aldiboronti, language hat, Syntinen Laulu.  The further elucidation you provide tends to reinforce my first thoughts.
Casa in Spanish indeed means a house, and mata is the third person singular, present tense, of the verb matar, to kill. But casamata sounds a bit like pidgin Spanish - an arbitrary cobbling together of two words. It might sound feasible to Anglo-Saxon ears, as a term meaning something like “killing house” - but not alogether so, to ears attuned to Spanish language and grammatical forms. A slaughterhouse in Spanish, for instance, is matadero; a bull-killer, matador. As OED says: “it is difficult on this theory to account for the form of the word” (it must be said, nevertheless, that the RAE does mention “ slaughter, destruction” as an archaic sense of mata).

Questions that arise: “Casemate” is a relatively new word in English - no citations before early 16th century. Is there good reason - indeed, any reason - to suppose that the term originated in any particular language? Are there antedates available in that language, to reinforce such a supposition?  Why Spanish or Italian? One early OED cite (1635 T. Heywood) spells the word “Chasemates”. Perhaps it’s originally French: Chasse- something (I don’t know French well enough to venture suggestions about the second part). Chasser (to hunt) has military connotations (cf. Chasseur Alpin). Of course this is pure speculation - but so, I think, are many of the explanations offered for “casemate” by the experts. I’m not trying to find fault, mind - the etymologists of the English language have undertaken a Herculean task, and one should not be surprised to find them brought up short on occasion. I think the etymology of “casemate” could do with some more research. Another question that comes to mind is: to what extent can one effectively research the etymology of English, without having also a considerable knowledge, not only of OE and ME, but of several other European languages too? Is such knowledge an academic requirement for etymologists?

FWIW, the Dutch cognate is ‘kazemat’. The EWN has an etymolgy more or less along the same lines as to what LH writes.

RHUD says from Italian casamatta, from Greek as above.

Garzanti on-line (Italian) says from “casa matta” = “false house”, no further.

ATILF on-line (French) shows both of these possibilities.

Swedish Academy dictionary on-line says from “casa”, otherwise obscure.

Kluge 2002 (German) says from “casa” + “matta”, with ‘regional’ “matta” equated to German “Erdscholle”, I guess “soil”: apparently an earth-covered house originally.

That is if I’m reading these right.

MW3 says from Italian casamatta, “probably from casa house + matta, feminine of matto mad, crazy, from Latin mattus stupid, drunk” (!).

Erdscholle = dirt clod, lump of earth.