Right in the centre of the main moat, this is the most significant of Muang Sema's ruins, after Wat Dhammachakra Semaram. Labelled as Monument 1 on the site's signs, it was from the Khmer era and would surely have been a typical prang (Khmer-style stupa), though now all that sits atop the pedestal is a yoni, the base for shiva lingas, and a somasutra, the channel in which holy water flowed out of the sanctuary after washing the main religious image.
The road leading to it starts just 500m from Wat Dhammachakra Semaram.