Frederick II brought overwhelming force to this strategic chokepoint. His army had marched north from Scania, while a fleet carrying 4,600 additional troops departed Copenhagen on August 5th. By August 22nd, Danish forces had completely encircled the fortress. No Swedish reinforcements could break through. No supplies would reach the garrison. The Danes simply waited, their cannon ready, their numbers decisive. When the bombardment finally began on September 1st, the outcome was already determined by logistics rather than courage.

The Swedish garrison defended with determination. Frederick II himself recorded incidents of Swedish resistance during the siege. But determination could not patch stone walls. After three days of concentrated artillery fire, Danish gunners had blasted a hole large enough for an assault. Commander Daniel Rantzau received orders to storm the breach. He never got the chance. Before Rantzau could lead his men through the gap, Erik Kagge raised the white flag. The fortress fell without a final stand, the Swedish commander perhaps recognizing that heroic defense would only add to the body count without changing the result.

Danish soldiers streamed into Alvsborg and stripped it bare. All 32 Swedish cannons became Danish property. Two warships anchored nearby were seized and absorbed into Frederick's fleet. Jorgen Rantzau took command of the conquered fortress with a garrison of four companies, later reduced to two as the war moved elsewhere. The Danish army turned south toward Halland to confront the forces of Eric XIV of Sweden. The loss of Alvsborg would haunt Sweden throughout the conflict, cutting off vital trade revenues and demonstrating the vulnerability of its western flank.

Alvsborg's strategic value extended far beyond its walls. The fortress commanded the route to Lodose, Sweden's window to western trade. Losing it meant losing access to the Atlantic at a time when maritime commerce determined national wealth. The Northern Seven Years' War would drag on until 1570, reshaping Scandinavian politics and borders. But the war's opening chapter, written in eleven days at this coastal fortress, showed how quickly geography could become destiny when the right chokepoint fell into enemy hands.