Mutianyu Section, Great Wall, Beijing, China — Built on a high ridge with 22 watchtowers in a 2.25-kilometer stretch, Mutianyu was a critical defensive line guarding the approaches to Beijing from the northeast. Its double-sided battlements (crenellations on both the inner and outer faces) allowed defenders to fight enemies who had already breached the wall.
© Ray in Manila, CC BY 2.0, via Wikimedia Commons
