We just learned about the hole in a castle wall called the Machicolation.
Another part of a castle is a Curtain Wall.
This is another wall that goes around the main castle, to help protect the castle inside the wall.
Its name makes sense because it's like a big curtain going around the outside of the castle.
(from: wikipedia - curtain wall (fortification))
Kid Facts - Blast from the past: Castel del Monte