In the 15th-16th century the settlement of Keila consisted some tens of buildings and a hundred people formed around the church. At the same time the Livonian Order built a small fort south-east of the church on Jõepark. During the Livonian War of 1558-1583 the settlement was destroyed.Further hampered by the plague and starvation in 1601-1602 the population declination reduced the community to a small church village. Ruins of the fort were first excavated in 1976 with continued finds up to 2007.
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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.