Mühlburg Erfurt, Weimar & Thuringia

The oldest of the three Drei Gleichen ('Three Like') castles, Mühlburg was first mentioned in 704 and possibly existed in some form as early as 319 (although the earliest dated remains are from the 13th century). While considered a ruin, its moat, walls, drawbridge and 22m-high tower are intact. There's a small museum displaying pottery and household items from the Middle Ages and you can climb the tower to take in splendid views over the adjacent town of Mühlberg and surrounding hills. Mühlburg is linked to the other two Drei Gleichen castles (Burg Gleichen and Veste Wachsenburg) by the 12km Burgenroute walking circuit.