Belváros (Inner City) Church is the oldest building in Pest side of river in Budapest. It was built in 1046 as a grave of Bishop St. Gellért (c. 980-1046), a missionary from Italy who played an instrumental role in converting Hungary to Christianity. According to tradition, he was martyred by angry pagans who rolled him down a hill across the river, which was named Gellért Hill in his honor.
The first church constructed on this spot was a 12th-century Romanesque structure built inside the ruined walls of the Roman fortress of Contra-Aquincum. In the early 14th century, after destroyed by Mongols, this church was replaced by a Gothic church which still stands today. It has been frequently renovated and remodeled in accordance with contemporary fashions, so its medieval origins are not obvious at first glance.
During the reign of Ottoman Empire Belváros was used as a mosque and in 1702 it was returned as a Christian church by Jesuits. The Rococo style pulpit dates from 1808.
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.