caponiere

from The Century Dictionary.

• noun In fortification: A covered lodgment sunk 4 or 5 feet into a ditch for its defense, encompassed with a parapet about 2 feet high, serving to support several planks laden with earth.
• noun A passage from one part of a work to another, protected on the right and left by a wall or parapet, and sometimes covered overhead. When there is a parapet on one side only, it is called a demi-caponiere.
• noun One of a series of bomb-proof arched structures for receiving cannon which fire through embrasures pierced in the front or mask-wall of the casemates: used for flanking ditches.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Fort.) A work made across or in the ditch, to protect it from the enemy, or to serve as a covered passageway.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military, historical A work made across or in the ditch, to protect it from the enemy, or to serve as a covered passageway.