The 'lower castle' served as a residence for the Counts and later The Princes of Celje during the 14th and 15th century. It was separated from the town settlement by battlements and a defence ditch. After the extinction of The princes of Celje in the 15th century, the court was used as an office of the Habsburg caretakers and was transformed into a barracks in the 18th century. The building we know today has undergone many radical changes, most of them during the reign of Empress Maria Theresa.
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.