In May 1626 King Gustavus Adolphus launched his invasion of Polish Prussia. Escorted by a fleet of over 125 ships, Swedish forces numbering over 8,000 soldiers (including 1,000 cavalry) disembarked in Ducal Prussia near Piława (Pillau). The landings were a complete surprise to the Polish–Lithuanian Commonwealth's defences, and despite his relatively small forces, Gustavus Adolphus quickly captured 16 Prussian towns, almost without a fight (with the neutral Duchy of Prussia's passive support). Many of these towns were inhabited by Protestants and they opened their gates freely to the Lutheran Swedish forces, who they saw as co-religionists. The Swedish king, however, failed to capture the largest prize - the city of Danzig (Gdańsk), which maintained its own small army and fleet for defence. In preparation for his major attack on Danzig, King Gustavus Adolphus increased his forces to over 22,000. The Polish king, Sigismund III, received no support from his vassal, Ducal Prussia, when fighting to defend it. He deployed north with an army of 11,000 men and pitched battle at Gniew against King Gustavus Adolphus' force of 8,150 infantry, 1,750 cavalry and 74 cannon. The fighting continued for several days - from 22 September to 1 October 1626 - until Sigismund III withdrew his army, and called on reinforcements from around the country.