The Sauerland with its gentle mountains, forests and lakes is a green oasis in this heavily populated region. The area is ideal for outdoor activities such as hiking and biking. One of the most picturesque Sauerland towns is Altena. The 12th century castle above the town has been well preserved and contains a museum featuring medieval weapons, armour and other artefacts. The Atta-Höhle (Atta Cave) near Attendorn is one of Germany's most incredible stalactite caves.