Within the fortification enclosure, the city’s bishop’s church is at the north-western end of the lower city, also known as the “Basilica of the Catechumens” or just “Forty Doors.” The tradition wants this church to be dedicated to St. Nicholas. However, Andreas Xygopoulos has expressed the opinion that the church was dedicated to St. Demetrius, taking into account the monumental representation of St. Demetrius on the south wall of the central nave of the church, where a dedicatory inscription of the Bishop of Servia, Michael, is still visible.

The church is a typical Middle Byzantine three-aisled basilica with a narthex and a raised central nave with a clerestory. The church’s central nave had a gable roof, while the side aisles and the narthex had separate single-pitched roofs. The naves were separated by walls, each with four arched openings. The middle aisle is twice as wide as the side aisles and is connected to the narthex by a triple opening, of which only the column bases remain. The great semicylindrical arch to the east of the church has now collapsed; its sturdy construction, however, prevented its complete destruction, so today, we can have a clear picture of its form, which is semicylindrical with shallow blind arches and a window.

The church dates from around the beginning of the 11th century but underwent successive phases of alterations from the 12th to the 16th century, the most characteristic being the 14th-century alteration in which the south aisle was converted into a portico.

No firm conclusions can be drawn about the sculptural decoration of the church from its few surviving examples. Two interesting sections of a marble epistyle still survive, which probably originated from the now-lost marble iconostasis of the church and were later used as spolia during an alteration of the basilica. They are decorated simply with birds pecking bunches of grapes. The relief’s rough finish has already led Andreas Xygopoulos to date the sculptures to the beginning of the 11th century, stating that they are ‘provincial clumsy imitations of a good original.’

The basilica bears interesting fresco decorations in at least two superimposed layers. The first, which was contemporary with the church’s construction, is covered by the second, which dates back to the 13th century and is visible today in a fragmentary state. The condition of the wall paintings is better in the church’s nave, as in the side aisles and the narthex, the paintings have been almost completely destroyed due to prolonged exposure to the weather and the sun. In the nave, the wall paintings are arranged in three zones: the upper zone, which is now totally indistinguishable, had ten large sections with representations, probably of the Twelve Great Feasts, while from the middle zone, we can still distinguish today, albeit with some difficulty, scenes from the Passion of Christ and the Presentation of the Virgin. The lower zone is the one that survives in better condition and includes full-length hierarchs, military and monastic saints, and female saints. Notable, though in a fragmentary state of preservation, are the representations of the Embrace of Saints Peter and Paul and the representation of the frontal full-length Saint Demetrius with the figures of the blessing Jesus and an angel handing him a sword, both of which are rendered on a smaller scale.