Mutianyu's 22 watchtowers run for 2.25 kilometers along a high ridge that once guarded the northeast approach to Beijing. Its double-sided battlements — crenellations on both inner and outer faces — allowed defenders to fight enemies who had already breached the wall. That detail tells you something about how seriously the Ming took the threat.
© GraceMarcellaNorman, CC BY 2.0, via Wikimedia Commons