The Church of St. Nicholas is an Eastern Orthodox Church in the Bulgarian capital of Sofia. The church was built in the 13th century by Kaloyan the sebastocrator as a small family church on today's Kaloyan Street behind the Rila Hotel. It is assumed that the church was erected on the site of the surviving parts of the palace complex that had once stood there which was built during the Roman Empire and accommodated Constantine the Great. Today these parts are preserved and can be seen in the authentic underground walls of the temple.
References:The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.