The explosion killed dozens of Danish soldiers in an instant. On May 22nd, 1612, with enemy troops pouring through a breach in Alvsborg's walls, Swedish defenders detonated charges at the fortress entrance. The blast stopped the Danish assault cold, sending survivors scrambling back to their siege lines. Christian IV of Denmark had expected his overwhelming force to take the castle quickly. Instead, Swedish commander Olof Andersson Strale and his garrison had turned desperation into a weapon. It was a tactical victory that could not change the strategic reality: Denmark would have the fortress within days.
The Kalmar War began over fish and furs. In April 1611, Christian IV declared war on Sweden after King Charles IX claimed sovereignty over Finnmark, the strategic Arctic region along the Atlantic-White Sea trade corridor. For generations, Danish-Norwegian monarchs had profited from Finnmark's resources. The Swedish claim threatened that revenue stream. Christian IV responded not with diplomacy but with soldiers, bringing his army north to seize Sweden's most valuable coastal asset: the fortress that controlled access to western trade routes.
Danish forces arrived in stages. On May 5th, 1612, the first portion of Christian IV's army reached Alvsborg and began the siege. By May 14th, the full force had assembled, surrounding the fortress completely. Christian opened bombardment and began digging approach trenches toward the walls. When he demanded surrender, the Swedish garrison offered a counterproposal: give them fourteen days, and if no reinforcements arrived, they would capitulate. Christian's response came in cannonballs. Swedish soldiers died in the subsequent barrage, but the garrison held.
By May 22nd, Danish artillery had punched through Alvsborg's walls. Christian IV ordered the assault. His soldiers charged through the gap, expecting to overwhelm the weakened garrison. They found instead a trap. Swedish defenders had prepared explosives at the entrance. The detonation inflicted heavy casualties on the attacking force, breaking the momentum of the assault. Danish troops retreated to regroup. The following day, Christian again demanded surrender. Strale asked for one more day to consider. He was buying time that had already run out.
On the morning of May 24th, Olof Andersson Strale led his remaining soldiers out of Alvsborg Fortress. The castle passed into Danish hands, where it would remain for the duration of the Kalmar War. Christian IV moved quickly to exploit his success. Within four days, a portion of his army had marched to Guldborg, another Swedish fortification. That castle's defenders, witnessing what had befallen Alvsborg, surrendered on June 1st after only token resistance. The western Swedish coast lay open to Danish control.
Located at 57.67N, 11.85E near the mouth of the Gota alv river at modern Gothenburg. The fortress site commands the waterway leading inland to historic Lodose. View from 2,000-4,000 feet for best perspective on the strategic position controlling the strait. Gothenburg Landvetter Airport (ESGG) lies 25nm to the east; Gothenburg City Airport (ESGP) is approximately 10nm north. The Kattegat coastline and river channels provide clear navigation references.