Most ranged attacks, including buildings, can hit stationary targets better than mobile targets. This fact makes cavalry particularly resistant to buildings, as they already have high pierce armor. This improvement allows your buildings to strike moving targets, including flying scouts or other flying units. 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.
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