The ruins of 'Lion City', the westernmost-known Khmer outpost, are spread around a 102-hectare compound girded by 4.5km of walls. Serving most likely as both a military post and a relay point for trade along the Mae Nam Khwae Noi, the restored ruins, built almost entirely of laterite, feature Bayon form, which would place it in the late 12th- or 13th-century under the Buddhist king Jayavarman VII.
There are two large main monuments and the remnants of two others. The principal shrine, Prasat Muang Sing (labelled Monument #1 on the site signs and maps), is in the centre and faces east (the cardinal direction of most Angkor temples). Inside are replica statues of the eight-armed Bodhisattva Avalokitesavara and the goddess Prajnaparamita; the latter was probably originally installed in the other large shrine behind this one. The ponds were probably used for religious purposes.
Outside the city wall are seven additional layers of ramparts and moats, which are visible at the main entrance. And right next to the river is a prehistoric burial site that shows two skeletons, pottery and jewellery thought to date back 2000 years.
Muang Sing is 33km west of Kanchanaburi. Tha Kilen train station (100B) is a 1.5km walk away, but it's best to come with your own transport since trains are infrequent and the grounds are large.