rows of stones and brickwork, and measures . Architecturally, it is a three-naved basilica church; the naves were formed by two rows of columns which ran along the length of the cella (or naos). It has three entrances, all from the west, and three apses in the easternmost part. The side apses are smaller than the one in the middle, which features an elaborate three-part window and additional decoration. The narthex and the four-stepped adobe synthronon (stone benches for the clergy) were added during the second period of construction in the 13th century. The synthronon fits inside the middle apse and includes a throne with railings in the middle. The church had an adjacent bell tower, which was a separate rectangular structure that lay to the southwest of the church. Its walls were long and around thick. Architects Vasilka Gerasimova–Tomova and Violeta Pesheva believe that it reached in height and that it was built in the 1210s under Bulgarian despot Alexius Slav (fl. 1208–1228), a largely independent medieval ruler of Melnik and the Rhodope Mountains. A bronze church bell with a relief Medieval Greek text, considered by Bulgarian researchers to be among the oldest extant church bells in Europe, was unearthed in Melnik in the 2000s. The inscription prompted the researchers to associate the bell with the Church of St Nicholas. It reads: "Copper-smelted church bell, a gift by Alexius, who is the pious Slav, to Saint Nicholas of Myra". The text is thought to reference Alexius Slav, thus it is dated to his rule and the construction of the bell tower. A very similar bell, also found in Melnik, bears an inscription which mentions Byzantine Emperor Michael VIII Palaiologos (r. 1259–1282) and the year 1270 specifically. However, this second bell may not have belonged to the Church of St Nicholas, but rather to another church in Melnik. The church's frescoes were painted in the 12th–13th century by three artists. The surviving murals include rare depictions of the ordination of Apostle James the Greater for bishop of Jerusalem by the Church Fathers and the vision of Peter of Alexandria. The life of church patron Saint Nicholas is also painted on the walls of the church. The frescoes also feature portraits of Antipas of Pergamum, Anthimus of Nicomedia, Gregory of Nyssa and Procopius of Scythopolis. Some of the murals have been stripped from the walls and sent to the National Archaeological