n. A trench dug in a moat to allow for drainage, or as an extra obstacle for attackers.
the GNU version of the Collaborative International Dictionary of English
n. A drain trench, in a ditch or moat; -- called also cuvette.
The Century Dictionary and Cyclopedia
n. In fortification: A deep trench sunk along the middle of a dry moat, to make the passage more difficult. A small drain dug along the middle of the main ditch, to receive the surface-water and keep the ditch dry.