San Gregorio is a former church founded in the 9th century and, in the 13th century, it became a Benedictine abbey. In the mid-15th century it was rebuilt to the current appearance under design by Antonio da Cremona. In 1775, after a long period of crisis, the monastery was closed and in 1807, after the Napoleonic occupation of Italy, also the parish church was suppressed. Deconsecrated, it was converted in a mint laboratory and then an art restoration center. It is now closed and not used.
It has a Gothic exterior with an ogival portal, elongated mullioned windows with ogival top, and apse.
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.