The old city walls, which once stretched for 9km around ancient Chángshā, were built of rammed earth in 202 BC, reinforced with stone in AD 1372, and finally demolished in 1928, save for this wonderfully imposing 250m-long section. You can enter Tiānxīn Park for free and wander around the old wall, but you have to pay to climb right up on top of it, and to visit the attractive Tiānxīn Pavilion which sits on top.
