Europe's abandoned castles are magnificent markers of the continent's tempestuous history. But where they once hosted royalty, these empty, hulking structures have now been 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 palace, explore the curious history of these imposing European structures and meet the creatures that call them home.