King Ladislaus V left Hungary for Vienna in May. Hunyadi hired 5,000 Hungarian, Czech and Polish mercenaries and sent them to Belgrade, which was the key fortress of the defense of Hungary's southern frontiers. The Ottoman forces marched through Serbia and approached Belgrade in June. A crusade made up mostly of peasants from the nearby counties, who had been roused by John of Capistrano's fiery oratory, also started to assemble at the fortress in the first days of July. The Ottoman siege of Belgrade, which was personally commanded by Sultan Mehmed II, began with the bombardment of the walls on 4 July.