Valletta, the capital of Malta, is one of the most concentrated historic areas in the world. Founded in 1566 by the Order of St. John following the Great Siege of Malta, it was the first pre-planned city in Europe. The city was designed on a grid system with high walls and deep moats to make it a nearly impregnable fortress. UNESCO has designated the entire city as a World Heritage site, noting its 320 monuments within just 55 hectares. Valletta is famous for its Grand Harbour, one of the deepest natural harbors in the world, and St. John's Co-Cathedral, which houses Caravaggio's only signed masterpiece. Despite its small size—it is the smallest capital in the European Union—it served as a vital strategic naval base during World War II, earning the entire island the George Cross for its bravery under heavy bombardment.