The passage of the Mincio obliged the Austrians to retire within the frontier of the Tyrol; and they might have been considered as completely expelled from Italy, had not Mantua and the citadel of Milan still continued to display the Imperial banners. The castle of Milan was a place of no extraordinary strength, the surrender of which might be calculated on so soon as the general fate of war had declared itself against the present possessors. But Mantua was by nature one of those almost impregnable fortresses, which may long, relying on its own resources, defy any compulsion but that of famine. The town and fortress of Mantua are situated on a species of island, five or six leagues square, called the seraglio, formed by three lakes, which communicate with, or rather are formed by, the Mincio. This island has access to the land by five causeways, the most important of which was in 1796 defended by a regular citadel, called, from the vicinity of a ducal palace, La Favorità. Another was defended by an intrenched camp, extending between the fortress and the lake. The third was protected by a horn-work. The remaining two causeways were only defended by gates and draw-bridges. Mantua, low in situation, and surrounded by water, in a warm climate, is naturally unhealthy; but the air was likely to be still more destructive to a besieging army, (which necessarily lay in many respects more exposed to the elements, and were besides in greater numbers, and less habituated to the air of the place,) than to a garrison who had been seasoned to it, and were well accommodated within the fortress. To surprise a place so strong by a coup-de-main was impossible, though Buonaparte represents his soldiers as murmuring that such a desperate feat was not attempted. But he blockaded Mantua [June 4] with a large force, and proceeded to take such other measures to improve his success, as might pave the way to future victories. The garrison was numerous, amounting to from twelve to fourteen thousand men; and the deficiencies of the fortifications, which the Austrians had neglected in over security, were made up for by the natural strength of the place. Yet of the five causeways, Buonaparte made himself master of four; and thus the enemy lost possession of all beyond the walls of the town and citadel, and had only the means of attaining the mainland through the citadel of La Favorità. Lines of circumvallation were formed, and Serrurier was left in blockade of the fortress, which the possession of four of the accesses enabled him to accomplish with a body of men inferior to the garrison.[155]