The goal of Trondheim's pilgrims in times past was the colossal Nidaros Domkirke at the south end of Munkegata (May to mid-June & mid-Aug to mid-Sept Mon– Fri 9am–3pm, Sat 9am–2pm, Sun 1–4pm; mid-June to mid-Aug Mon– Fri 9am–6pm, Sat 9am–2pm, Sun 1–4pm; mid-Sept to April Mon– Fri noon–2.30pm, Sat 11.30am–2pm, Sun 1–3pm; 40kr, includes entry to Erkebispegården coronation museum). Scandinavia's largest medieval building, it has been gloriously restored following several fires and the upheavals of the Reformation, it remains the focal point of any visit to the city and is best explored in the early morning, when it's reasonably free of tour groups. The building, still known by Trondheim's former name, is dedicated to King – later Saint – Olav. Born in 995, Olav Haraldsson followed the traditional life of the Viking chieftain from the tender age of 12, "rousing the steel-storm" as the saga writers put it from Finland to Ireland. He also served as a mercenary to both the duke of Normandy and King Ethelred of England, and it was during this time that he was converted to Christianity. In 1015 he invaded Norway, defeated his enemies and became king, though his zealous imposition of Christianity alienated many of his followers and the bribes of Olav's rival Knut (Canute), King of England and Denmark, did the rest: Olav's retainers deserted him and he was forced into exile in 1028. Two years later, he was back in the Trøndelag, but the army he had raised was far too weak to defeat his enemies, and Olav was killed in battle near Trondheim.

Olav may have lost his kingdom, but the nationwide church he founded had no intention of losing ground. Needing a local saint to consolidate its position, the church carefully nurtured the myth of Olav, a beatification assisted by the oppressive rule of the "foreigner" Knut. After the final battle, Olav's body had been spirited away and buried on the banks of the River Nid at what is today Trondheim. There were rumours of miracles in the vicinity of the grave and, when the bishop arrived to investigate these strange goings-on, he exhumed the body and found it, lo and behold, perfectly uncorrupted. Olav was declared a saint, his body was placed in a silver casket and Olav Kyrre, who became King of Norway in 1066, started work on the grand church that was to house the remains in appropriate style. Over the years the church was altered and enlarged to accommodate the growing bands of medieval tourists; it achieved cathedral status in 1152 and subsequently became the traditional burial place of Norwegian royalty.

The cathedral is a magnificent blue- and green-grey soapstone edifice, with a copper-green spire and roof and a fancy set of gargoyles on the choir. It's also a true amalgam of architectural styles. The original eleventh-century church was a simple basilica, but subsequent alterations enlarged it considerably. The Romanesque transepts, with their heavy hooped windows and dog-tooth decoration, were built by English stonemasons from Lincoln in the twelfth century, while the early Gothic choir, with its flying buttresses and intricate tracery, is clearly influenced by contemporaneous churches in England. The nave was built in the early thirteenth century, also in the early Gothic style, but was destroyed by fire in 1719; the present structure is a painstakingly accurate late nineteenth-century replica.

Inside, the gloomy half-light hides much of the lofty decorative work, but it is possible to examine the striking choir screen, whose wooden figures are the work of Gustav Vigeland. The other item of particular interest is a famous fourteenth-century altarfrontal (the front panel of an altar painting) displayed in a chapel off the ambulatory, directly behind the high altar. This is the earliest surviving representation of Olav's life and times, created during a period when few Norwegians could read or write and the saint's cult had to be promoted visually.

What you won't see, however, is St Olav's silver casket-coffin, which was taken to Denmark and melted down for coinage in 1537; but you can see the Norwegian crown jewels in the new coronation museum next to the Archbishop's palace. The lavish display of crowns, orbs, sceptres and the anointing horn is a permanent fixture, as coronations were abolished in 1908.

Two other features are the English-language guided tours of the cathedral (mid-June to mid-Aug at 11am, 1.30pm, 3pm & 4pm; 30min), and the climb up the cathedral tower (every half-hour late June to late Aug Mon– Fri 10am–5pm, Sat 10am–12.30pm, Sun 1–3.30pm). From the top, there's a fine view of the city and the forested hills that surround it, with the fjord trailing away in one direction, the river valley in the other.