There have been some fortifications in Lappeenranta city from the 17th century. After the defeat of Sweden-Finland in Great Northern War 1700-1721 Viborg castle and large areas in Carelia were lost to Russia. The military value of Lappeenranta, the new border city, was suddenly increased. The construction of the new bastion fortress was started immediatelly after war in 1721. It was planned to be a part of the new defence system of Finland together with Olavinlinna castle and Hamina fortress.
Russians conquered the fortress in the next war in 1741 after bloody battle. New border was moved to west and Lappeenranta was left to Russian side. Russians settled a garrison to fortress and started to enhance it in the 1750's and again in 1791-1792. After the Finnish War in 1808-1809 fortress lost its military value. After year 1810 it was used as a garrison and prison. During the Finnish Civil War in 1918 red guards were arrested and executed in Lappeenranta fortress.
Nowadays fortress has been renovated and open for visitors. The former garrison buildings are now home to the South Karelian Museum and Art Museum, artists' and craft workshops, Lappeenranta's Orthodox church and parish hall, a children's art school and a café, among others.
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.