Bishop Arnaldur (1124-52) returned to Norway in 1150 from Gardar, Greenland and was appointed first Bishop of Hamar. He began to build the cathedral, which was completed about the time of Bishop Paul (1232-52). Bishop Thorfinn of Hamar (1278-82) was exiled and died at Ter Doest in Flanders. Thorfinn and many other bishops of the area disagreed with the sitting King Eric II of Norway regarding a number of issues, including episcopal elections. Bishop Jörund (1285-86) was transferred to Trondhjem.
In the aftermath of the Reformation in Norway, the structure was renamed Hamarhus fortress and made into the residence of the sheriff. The cathedral was still used but fell into disrepair culminating with the Swedish army’s siege and attempted demolition in 1567, during the Northern Seven Years' War. Swedish forces had launched attacks into Eastern Norway, capturing Hamar and continued towards Oslo. The Swedes later retreated, torching Hamar on their way, destroying Hamar Cathedral and Hamarhus.
Today the ruins of Hamar Cathedral form a part of the Hedmark museum (Hedmarksmuseet). The ruins of what remain of the Hamar cathedral, were originally built in Romanesque architecture and later converted to Gothic architecture. The distinctive arches in the cathedral ruins are covered in one of the most ambitious construction projects of its kind undertaken by the Norwegian government.
References:The Basilica of Santa Maria in Trastevere is one of the oldest churches of Rome. The basic floor plan and wall structure of the church date back to the 340s, and much of the structure to 1140-43. The first sanctuary was built in 221 and 227 by Pope Callixtus I and later completed by Pope Julius I.
The inscription on the episcopal throne states that this is the first church in Rome dedicated to Mary, mother of Jesus, although some claim that privilege belongs to the Basilica di Santa Maria Maggiore. A Christian house-church was founded here about 220 by Pope Saint Callixtus I (217-222) on the site of the Taberna meritoria, a refuge for retired soldiers. The area was made available for Christian use by Emperor Alexander Severus when he settled a dispute between the Christians and tavern-keepers.
The church underwent two restorations in the fifth and eighth centuries and in 1140-43 it was re-erected on its old foundations under Pope Innocent II.