The Collaborative International Dictionary
Counterscarp \Coun"ter*scarp`\ (koun`t[~e]r*sk[aum]rp`) n. [Counter- + scarp: cf. F. contrescarpe.] (Fort.) The exterior slope or wall of the ditch; -- sometimes, the whole covered way, beyond the ditch, with its parapet and glacis; as, the enemy have lodged themselves on the counterscarp.
Wiktionary
n. The outer slope of a defensive or fortified ditch
Wikipedia
A scarp and a counterscarp are the inner and outer sides of a ditch or moat used in fortifications. Attackers (if they have not bridged the ditch) must descend the counterscarp and ascend the scarp. In permanent fortifications the scarp and counterscarp may be encased in stone. In less permanent fortifications, the counterscarp may be lined with paling fence set at an angle so as to give no cover to the attackers but to make advancing and retreating more difficult.
If an attacker succeeds in breaching a wall a coupure can be dug on the inside of the wall to hinder the forlorn hope, in which case the side of the ditch farthest from the breached wall and closest to the centre of the fortification is also called the counterscarp.
Usage examples of "counterscarp".
Ostend had been completely rebuilt and fortified eighteen years previously, and was defended by ramparts, counterscarps, and two broad ditches.
Davenport summoned them all down the steep counterscarp, or exterior slope of the ditch.
So mighty-looking is this vehicle that you can't think of it in the language of designers or engineers, but must resort to the vocabulary of military architecture, because in spite of its sleekness, it seems to be a fortress on wheels: all compact buttresses, ramparts, terrepleins, scarps, counterscarps, bastions made aerodynamic, condensed and adapted to rolling stock.