Before Hunyadi could assemble his forces, the army of Mehmed II  arrived at Belgrade. The siege began on July 4, 1456. Szilágyi could rely on a force of only 5,000-7,000 men in the castle. Mehmed set up his siege on the neck of the headland and started heavily bombarding the city's walls on June 29. He arrayed his men in three sections: The Rumelian corps had the majority of his 300 cannons, while his fleet of 200 river war vessels had the rest of them. The Rumelians were arrayed on the right wing and the Anatolian corps were arrayed on the left. In the middle were the personal guards of the Sultan, the Janissaries, and his command post. The Anatolian corps and the Janissaries were both heavy infantry troops. Mehmed posted his river vessels mainly to the northwest of the city to patrol the marshes and ensure that the fortress was not reinforced. They also kept an eye on the Sava river to the southwest to avoid the infantry from being outflanked by Hunyadi's army. The zone from the Danube eastwards was guarded by the Sipahi, the Sultan's feudal heavy cavalry corps, to avoid being outflanked on the right.
Question: Which event happened first, the siege, or Hunyadi assembling his forces?
Answer: The siege