Structure and age of ancient walls from the city of Nan, northern Thailand

The structure and age of remains of two ancient walls located in the city of Nan, a former part of the Lanna Kingdom in modern northern Thailand, were investigated. The remains differ in appearance with a well-preserved brick wall present at Mahawong Road (MHW), while an earthwork with a brick reinforcement core is found at Phaya Wat Temple (PWT). We employed an electrical resistivity tomography (ERT) survey and optically stimulated luminescence dating (OSL) to determine the buried structure of the earthwork at the PWT site and the production age of bricks from both locations. The ERT results reveal lateral heterogeneous resistivity domains cross-cutting the wall, which presumably relates to the construction of a dam. According to OSL dating, the PWT wall was likely built during the time of the war with Burma that ultimately resulted in a foreign occupation of Nan in the sixteenth century. This structure played a role in both flood mitigation and military defence of the city. The structure at MHW represents a city wall that was built at the end of the nineteenth century after Nan was relocated to its present position and rather reflects representative and social needs.