Castle Island is an island off the western coast of the LEGOLAND state of Castle Cove. After Basil the Bat Lord and the Fright Knights began a mass takeover of Castle Cove, a group of Lion Knights led by a knight named Leo fled to Castle Island and built their own castle. However, they were followed by the Bulls, allies of the Fright Knights, who constantly waged war with the Lions.

As the Great Castle Cove War pressed on, the Fright Knights and Ancient Knights each established their own castles on the island. The war reached its climax on Castle Island in 2007 with the defeat of Cedric and Vladek on Castle Island. Today, only the Bulls, Lions, and Ancient Knights have castles on the island; the Fright Knights' castle has since been destroyed.