During the time while Peter was thus employed in laying the foundations of his new city, the King of Sweden was carrying on the war in Poland against the conjoined forces of Russia and Poland, which were acting together there as allies. When intelligence was brought to him of the operations in which Peter was engaged on the banks of the Neva, he said, "It is all very well. He may amuse himself as much as he likes in building his city there; but by-and-by, when I am a little at leisure, I will go and take it away from him. Then, if I like the town, I will keep it; and if not, I will burn it down." [Illustration: Situation at St. Petersburg.] Peter, however, determined that it should not be left within the power of the King of Sweden to take his town, or even to molest his operations in the building of it, if any precautions on his part could prevent it. He had caused a number of redoubts and batteries to be thrown up during the summer. These works were situated at different points near the outlet of the river, and on the adjacent shores. There was an island off the mouth of the river which stood in a suitable position to guard the entrance. This island was several miles distant from the place where the city was to stand, and it occupied the middle of the bay leading toward it. Thus there was water on both sides of it, but the water was deep enough only on one side to allow of the passage of ships of war. Peter now determined to construct a large and strong fortress on the shores of this island, placing it in such a position that the guns could command the channel leading up the bay. It was late in the fall when he planned this work, and the winter came on before he was ready to commence operations. This time for commencing was, however, a matter of design on his part, as the ice during the winter would assist very much, he thought, in the work of laying the necessary foundations; for the fortress was not to stand on the solid land, but on a sandbank which projected from the land on the side toward the navigable channel. The site of the fortress was to be about a cannon-shot from the and, where, being surrounded by shallow water on every side, it could not be approached either by land or sea. Peter laid the foundations of this fortress on the ice by building immense boxes of timber and plank, and loading them with stones. When the ice melted in the spring these structures sank into the sand, and formed a stable and solid foundation on which he could afterward build at pleasure. This was the origin of the famous Castle of Cronstadt, which has since so well fulfilled its purpose that it has kept the powerful navies of Europe at bay in time of war, and prevented their reaching the city. Besides this great fortress, Peter erected several detached batteries at different parts of the island, so as to prevent the land from being approached at all by the boats of the enemy.