Quad'rilat'eral, four fortresses of northern Italy — Mantua, Verona, Peschiera and Legnano — which form a sort of outwork to the bastion of the mountains of the Tyrol, and divide the plain north of the Po into two sections by a powerful barrier. They have figured in all the later wars fought in northern Italy, especially in the wars between Austria and the different Italian states. Russia has a similar combination in Poland, called the Polish Quadrilateral.