Corregidor was not a fortress island for nothing - it's armed to the teeth. The United States spent hundreds of millions of dollars fortifying its defenses just before the war. In fact, it and 3 other islands in the Bay (Caballo, El Fraile and Carabao Island), plus Grande Island in Subic Bay in the north, constitute the harbor defences of Manila and Subic.
Scattered all over the island are 23 batteries - named after US servicemen - such as Battery Crockett and Battery Geary. These batteries are self-contained structures with 14-inch anti-ship artillery and their own magazines and ammunition stores.
Some of these batteries were destroyed, such as this one below, which suffered a direct hit from a Japanese bomber.