The Byzantine Church of Panagia Katholiki at Gastouni is a two-columned inscribed cruciform church dating from the 12th century. It has a semi-hexagonal apse, dome with eight-sided drum and a narthex at the west side. The walls, with panels of reticulate tiling and double layers of tiles at the joins, are embellished with a plain ceramic decoration, especially dentil courses, and with embedded glazed bowls. The unique Frankish influence of the 14th century is seen in the pointed arch of the north doorway, now blocked.
The later addition of an exonarthex (outer porch) and large doorway openings are the only few changes that have been made to the original form of the building. Two layers of wall paintings are preserved on the interior. The second layer is dated by an inscription to the year 1702 and was made with funds contributed by Ioannikios the bishop of Olene. A second inscription of the same period referring to the donors of the church and giving the date of building as 1279, refers in all likelihood to the earlier donors' inscription.