The oldest parts of Gerum Church are the choir and apse, dating from circa 1200 and Romanesque in style. The presently visible, Gothic nave dates from a later time of the 13th century and probably replaced an earlier, Romanesque nave. The tower, which was never finished, was built circa 1300. The only non-medieval part of the church is the sacristy, built in 1835.
Gerum Church is constructed of limestone. The exterior is whitewashed apart from several finely carved stone details. The façade is broken by four windows on the southern side while the northern completely lacks windows. The church has three portals, one Romanesque in the choir and two Gothic in the nave and tower, respectively. The Gothic portals are decorated with stone sculptures. Inside, the church carries frescos from at least three different periods: the 13th century, the 15th century (by the Master of the Passion of Christ) and the 18th century. The church also has a decorated stained glass window from the 14th century.
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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.