Crenellations is a common technology in Age of Mythology. It improves the accuracy of defensive buildings against moving targets and tower attack against cavalry. To enable this technology, the Watch Tower technology must first be researched.
Shortly after the development of stone walls, ancient architects figured out the utility of piercing defensive walls with arrow slits and crowning the walkways on top of the walls with crenellations that could protect an archer while still allowing them a wide field of view. The citadel of Buhen was adorned with hundreds of curved crenellations that were typical of Egyptian walls.