We don't have synonyms for caponier yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are caponizer, caponize and capone.
(n) :
(military, historical) A work made across or in a ditch, to protect it from the enemy (as fire from it could cover the ditch), or to serve as a covered passageway (e.g. to outworks).
(n) :
(military, historical) A work made across or in a ditch, to protect it from the enemy (as fire from it could cover the ditch), or to serve as a covered passageway (e.g. to outworks).