Waal is a term that has been used in German-speaking countries since the Middle Ages for the moats and ramparts of castles, today mostly small, abandoned castles of the lower nobility, such as tower hill castles and small moated castles. Alternative names are: Inselteich, Wallteich, Walteich, Wallgraben, Wohl, Wol, Wohlteich, Burgstall, Purgstall, Ringwall or Bühl. Originally, the term Waal is said to have only referred to elevations, i.e. the castle hill itself (and possibly the ramparts), but over the centuries it has come to mean all the remains of abandoned castles, i.e. also their moats/moats and the ramparts as well as the castle hill. The early modern term Wall (fortification, field fortification, city fortification) is presumably derived from Waal, as the alternative names Wallteich and Walteich show. Waal also refers to ponds with islands that once had moated castles or defensive towers.
Source: Wikipedia