The old city walls, which once stretched for 9km around ancient Changsha, were built of rammed earth in 202 BC, reinforced with stone in AD 1372 (during the Ming dynasty), damaged by the Taiping in 1852 and finally demolished in 1928, save for this imposing 251m-long section. You can enter Tianxin Park for free and wander around the old wall, but you have to pay to climb up on top of it, and to visit the attractive Tianxin Pavilion atop it.
The extant section was restored in 1983.