Passage: Zrinski did not allow the final assault to break into the castle.  As the Turks were pressing forwards along a narrow bridge the defenders suddenly flung open the gate and fired a large mortar loaded with broken iron, killing 600 attackers. Zrinski then ordered a charge and led his remaining 600 troops out of the castle. He received two musket wounds in his chest and was killed shortly afterwards by an arrow to the head. Some of his force retired into the castle.The Turks took the castle and most of the defenders were slain. A few of the captured defenders were spared by Janissaries who had admired their courage, with only seven defenders managing to escape through the Ottoman lines. Zrinski's corpse was beheaded and his head taken to the new Sultan while his body received an honourable burial by a Turk who had been his prisoner, and well treated by him.

Question: How many attackers were killed when the gate opened?
600