In spring 1197, Sverre called out the leidang from the northern and western parts of the country, and in May he was able to sail south to Viken with more than 7000 men, a considerable force. The Birkebeiners attacked Oslo 26 July, and after many casualties on both sides, the Baglers were forced inland. Sverre now spent some time war-taxing the region, but with his leidang troops close to mutiny, Sverre withdrew to Bergen where he had decided to spend the winter. This was to be a near fatal mistake. The Baglers had meanwhile travelled north to Trøndelag by land where they had entered Nidaros with little opposition. The garrison at Sverresborg held fast for a while until their commander Torstein Kugad changed sides and let the Baglers into the castle. The Baglers had Sverresborg completely dismantled. Sverre's home region was now in enemy hands.