Situated outside the town wall to the south of Nakhon Chum, this square fortress was made of laterite, with sides of 84 metres, and gates on each side. The northern side is completely destroyed. The fortress was fortified by battlements and enclosed by parapets with openings for defence. Watchtowers projected from the four corners, and small openings in the lower part of the walls allowed entry. Considering the similarity between these small entrances and window openings from the Ayutthaya period (starting from the reign of King Narai the Great), it is possible that Thung Setthi fortress was built around the middle Ayutthaya period, under the influence of European architecture.