Waal is a term that has been used in German-speaking countries since the Middle Ages for the ditches and ramparts of castles, today mostly small 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 was understood to mean the entire remains of lost castles, including their moats/moats and the ramparts and the castle hill. The early modern term wall (fortress construction, field fortification, city fortification) presumably derives from Waal, as the alternative names Wallteich and Walteich show. Waal also refers to ponds with islands that once had moated castles or defense towers.
Source: Wikipedia