What is the name of the slits in the walls on a castle?

The slits are simply called arrow slits or occasionally Balistraria. Sometimes the arrow slits are looking out of embrasures. Arrow slits were useful for protecting the castle as they were easy to fire out of but very, very hard to fire into. To find out more about embrasures click on the link below. "Wikipedia.org: Embrasure" To find out more about arrow slits click on the other link below. "Wikipedia.org: Arrow slits".