Standing deep in the German countryside for over 500 years, Heidelberg Castle was once home to the country's first university. Now, it has become part of the surrounding ecosystem. From toads swimming in the grounds' fountains to peregrine falcons nesting in the bell tower, tour a medieval mausoleum steeped in nature.

Predjama Castle is carved into the heart of a Slovenian mountain and nestled among stunning natural attractions, including the eerie Postojna Caves and Lake Cerknica--Europe's largest intermittent lake. Brown bears, aquatic salamanders known as olms, and more thrive in the shadow of this imposing fortress.