As the siege went on, Gustavus, writing under his enforced inaction, pleaded hard with the two Electors. From the Elector of Brandenburg he demanded the right to occupy the two fortresses of Küstrin and Spandau. Hopes were held out to him of the surrender of Küstrin, but he was assured that Spandau should never be his. Accompanied by a picked body of troops, he marched straight upon Berlin. On May 13, outside the city gates, he held a long conference with his brother-in-law, the Elector. He argued in vain. To one of the Dukes of Mecklenburg, who had accompanied him, he spoke in bitter words. "I am marching," he said, "upon Magdeburg, to deliver the city. If no one will assist me, I will retreat at once. I will offer peace to the Emperor, and go home to Stockholm. I know that the Emperor will agree to my terms. But you Protestants will have to answer at the day of judgment that you would do nothing for the cause of God. In this world, too, you will be punished. Magdeburg will be taken, and, if I retire, you will have to look to yourselves." The next day the conference was resumed. From early morning till nine at night the Elector persisted in his refusal. But the armed men who stood behind Gustavus were the most powerful of arguments. At last the Swedish king had his way. On the 15th the gates of Spandau were thrown open to his troops. [Sidenote: § 8. Resistance of the Elector of Saxony.] But, if the Elector of Brandenburg had given way, the Elector of Saxony was not to be moved. He had not yet received an answer to his appeal to the Emperor; and till that arrived he would enter into no alliance with a foreigner. Further advance was impossible. Cut to the heart by the refusal, Gustavus withdrew, leaving Magdeburg to its fate. [Sidenote: § 9. Storming of Magdeburg.]