Diocese of Nidaros in the context of Central Norway

Trondheim (UK: /ˈtrɒn(d)haɪm/ TRON(D)-hyme, US: /ˈtrɒnheɪm/ TRON-haym; Urban East Norwegian: [ˈtrɔ̂nː(h)æɪm]; Southern Sami: Tråante), historically Kaupangen, Nidaros, and Trondhjem (locally [ˈtrɔ̂nːjæm] ), is a city and municipality in Trøndelag county, Norway. As of 2025, it had a population of 216,518. Trondheim is the third most populous municipality in Norway, and is the fourth largest urban area. Trondheim lies on the south shore of Trondheim Fjord at the mouth of the River Nidelva. Among the significant technology-oriented institutions headquartered in Trondheim are the Norwegian University of Science and Technology (NTNU), the Foundation for Scientific and Industrial Research (SINTEF), the Geological Survey of Norway (NGU), and St. Olavs University Hospital.

The settlement was founded in 997 as a trading post and served as the capital of Norway through the Viking Age until 1217. From 1152 to 1537, the city was the seat of the Catholic Archdiocese of Nidaros; it then became, and has remained, the seat of the Lutheran Diocese of Nidaros and the site of the Nidaros Cathedral. The city was incorporated in 1838. The current municipality was formed in 1964, when Trondheim merged with Byneset Municipality, Leinstrand Municipality, Strinda Municipality, and Tiller Municipality, and further expanded on 1 January 2020, when Trondheim merged with Klæbu Municipality.

Nidaros, Niðarós or Niðaróss (Old Norse pronunciation: [ˈniðɑˌroːsː]) was the medieval name of Trondheim when it was the capital of Norway's first Christian kings. It was named for its position at the mouth (Old Norse: óss) of the River Nid (the present-day Nidelva).

Although the capital was later moved to Oslo (around the year 1300), Nidaros remained the centre of Norway's spiritual life until the Protestant Reformation. The Archdiocese of Nidaros was separated from Lund (in Scania) by the papal legate Nicholas Breakspeare in 1152, and the shrine to Saint Olaf in Nidaros Cathedral was Northern Europe's most important pilgrimage site during the Middle Ages. Archbishop Olav Engelbrektsson led Norway in its attempted resistance against the Danish Reformation, and was forced into exile by King Christian III in 1537. The archdiocese was abolished and replaced with a Lutheran diocese.

An octagonal church has an octagonal (eight-sided polygon) architectural plan. The exterior and the interior (the nave) may be shaped as eight-sided polygon with approximately equal sides or only the nave is eight-sided supplemented by choir and porch (or narthex) attached to the octagon. This architectural plan is found in some 70 churches in Norway. Among these Hospitalskirken in Trondheim is the oldest. This type of church plan spread from the Diocese of Nidaros to other parts of Norway. Virtually all octagonal churches in Norway are constructed as log buildings mostly covered by clapboards. Some of the largest churches in Norway are octagonal and the list includes important cultural heritage monuments such as Trinity Church (Oslo), Sør-Fron Church, and Røros Church.