The church of Koimisi Theotokou (Assumption of Mary) is located at the central square of Kastania and is the parish church. The original building, dating to the late 13th century, is a domed two-column cross-in-square, with an octagonal dome and triple apse at the east. The addition of a narthex in later years and the subsequent extension of the building westward gave the interior of the church the form of a three-aisle basilica.

The double layer, arched bell tower at its west face is also a later addition. The original church was built with the cloisonné technique and has elaborate brick decoration, while the later additions were built with rubble masonry. Sculpted panels have been immured in the drum of the north arm and the frame of the south entrance. Immured plates have been preserved in the walls and the dome. In the interior, the capitals of the eastern pair of columns bear relief decoration with original motifs, dating to the 12th century.