The rock church S. Matteo dell'Arena is located in the moat of the wall; so called because it is washed by the sea.
According to Finamore Pepe, it was a temple dedicated to Neptune. In the modern age, it became a depot in the service of the soap maker. Cleaning works have allowed the interior to be studied, although the three entrances from the sea side have not been identified.
The plan is an irregular quadrilateral. The interior has two naves, the widest of which is at the apse. Two steps elevate the bema from the aisles.
The altar, located in the apse, consisted of a lithic parallelepiped.
Of the extensive pictorial cycle, frescoes are still present, legible even if abraded.