The siege was the final act of a drama that had been building for years. Olav Engelbrektsson, Norway's last Catholic Archbishop, had constructed Steinvikholm Castle between 1525 and 1532 precisely for a moment like this -- a fortified refuge where the Church could resist the encroachment of Danish Protestant rule. When King Christian III of Denmark moved to impose the Reformation across his domains, Engelbrektsson had gathered the most sacred objects from Nidaros Cathedral, including the shrine of St. Olav, and brought them to the castle for safekeeping. But by the spring of 1537, the archbishop's position had become untenable. On Easter Day, 1 April, he boarded a ship and sailed for exile in the Netherlands, leaving his garrison to face the Protestant forces alone.

The siege began shortly after the archbishop's departure. Noble Tord Roed led the Protestant besieging force, which established a naval blockade of the fjord approaches while ground troops invested the island from the Skatval peninsula. The castle sat on a rocky island with no fresh water supply of its own -- everything had to come from the mainland, and the blockade cut that lifeline. The defenders answered with everything the castle's guns could deliver, firing their cannons at the besiegers around the clock. Multiple demands for surrender were refused. The garrison's resolve held through weeks of bombardment and blockade, the sound of cannon echoing across the fjord waters. Then a rumor reached them that changed everything. Word spread that the Danish noble Truid Ulfstand was sailing from Denmark with a force of 1,500 men. Whether the defenders feared this fresh army or simply recognized that reinforcement from the Catholic side would never come, the effect was the same. On 17 May 1537, Steinvikholm's garrison laid down their arms.

The surrender terms the defenders negotiated were remarkably generous, a testament to the pragmatism that often governed Reformation-era conflicts. Knud Pederson Skanke stipulated that no defender would be punished for their role in the rebellion, that all would receive pardons, and that he himself would retain his possessions and his position as deacon. The besiegers accepted every condition. On 29 May, the nobles Truid Ulfstand and Christoffer Huitfeldt formally ratified the agreement. The leniency was strategic as much as merciful -- the Protestant authorities needed stability in Trondelag, not martyrs. With Steinvikholm subdued, Catholic resistance in Trondelag and Northern Norway collapsed entirely.

The fall of Steinvikholm did not immediately end all Catholic resistance in Norway. Pockets of opposition lingered in parts of Eastern Norway, and in June 1537, Truid Ulfstand marched south to lay siege to Hamarhus, another Catholic stronghold. But the strategic significance of Steinvikholm's capitulation was decisive. It extinguished the last organized military resistance to the Reformation in Norway's heartland. The treasures the archbishop had gathered at the castle -- including the silver-sheathed shrine of St. Olav -- fell into Danish hands and were eventually destroyed or melted down. The castle itself, stripped of its purpose, slowly fell into ruin. Today its walls stand as a monument to the moment Norway's medieval religious order ended, not with a great battle but with a negotiated surrender on a rocky island in the fjord.