Gråborg is the largest ancient castle in Öland. It was built probably in the 6th century and enlarged through Middle Ages. According old tax reports dating back to the year 1450, Gråborg was owned by Vadstena abbey and functioned as a some kind of trade center. It was used for defence against Danish even in 1677. According to legend Gråborg was strongly associated with king Burislev Sverkersson who had grown up there with his half-sister Sofia of Minsk.
Gråborg Castle consisted of a circular wall, about 4-7 meter tall. There were three entrances and one of them was vaulted in the Middle Ages. Archaeological excavations have revealed lot of findings, which are stored today in Kalmar Museum.
There are medieval ruins of the chapel of St. Knut on the north side of castle.
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.