The abandoned castles of Europe are magnificent markers of the continent's tempestuous history. But where they once hosted royalty, these empty, hulking structures are now overtaken by the wildlife and rich natural environments flourishing around them. From peregrine falcons ruling the bell towers of Germany's Heidelberg Castle, to exotic amphibians thriving in the spring-fed fountains of Spain's iconic Alhambra, explore the curious history of these imposing European structures and the creatures that call them home.