Standing deep in the German countryside for over 500 years, Heidelberg Castle was once home to the country's first university and is now part of the surrounding ecosystem.
North of the Pyrenees stands the ancient Carcassonne, a stone fortress that's faced sieges, endured the brutality of the Spanish Inquisition, and thrived as a center of wealth during the salt trade a millennia ago.
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.
In the sun-baked plains of southern Spain, between Europe's only desert and the mighty Sierra Nevada, lies Alahambra, a magnificent Moorish palace of lush gardens and shimmering fountains.