A machicolation is a floor opening between the supporting corbels of a battlement, through which stones an lethally hot liquids and substances could be dropped on attackers at the base of a defensive wall. The design was developed in the Middle Ages when the Norman crusaders returned. A machicolated battlement projects outwards from the supporting wall in order to facilitate this. A hoarding is a similar structure made of wood, usually temporarily constructed in the event of a siege. One advantage of the machicolation over wooden hoarding is protection behind stone battlements, as well as being fire proof.