St. Clement's Church or Klemet's Church was one of the Roman Catholic parish churches of the old Oslo. It was a stone church with a tower, and it was one of the very few churches we know with the double-nave floor plan. Along the middle axis of the choir there were three powerful pillars that held the roof. The church went out of use after the Protestant Reformation, and expect it rather quickly was in ruins.
The church was exposed and examined through excavation by arcitect Johan Adolf Gerhard Fischer in 1921. Then remained for years unnoticed during Loeng bridge. In 1970-71 was archaeologist Ole Egil Eide opportunity to dig further into the ground under the church, and found traces of burials older than the stone church, 81 in all. His interpretation is that there have been at least two churches, presumably stave churches, on the spot where the stone church was built around 1100. The oldest of the graves are dated to the radiological 980-1030, and were some of the oldest Christian burials found in Norway. Ruins of Clement's Church is now exposed and is included as part of Middelalderparken in the Old Town of Oslo.
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.