A parapet originally meant a defensive mini-wall made of earth or stone that was built to protect soldiers on the roof of a fort or a castle. Now it indicates any low wall along the roof of a building, the edge of a balcony, the side of a bridge, or similar structure.
When drawing castles, the most remarkable feature is the parapet on the top of the towers — the notches in the parapet were designed for soldiers to stand in but have come to look like a larger version of the crown of the king inside. Now, parapets are more likely to keep you from falling off a roof than protect you from enemy soldiers trying to storm the building.
