We don't have synonyms for casemate yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are classmate, castrate and catenate.
(n) :
a bombproof chamber, usually of masonry, in which cannon may be placed, to be fired through embrasures; or one capable of being used as a magazine, or for quartering troops; a hollow molding, chiefly in cornices
(n) :
a bombproof chamber, usually of masonry, in which cannon may be placed, to be fired through embrasures; or one capable of being used as a magazine, or for quartering troops; a hollow molding, chiefly in cornices