Arnstein ruins, Medieval castle ruins in Maria Raisenmarkt, Austria.

The Arnstein ruins are the remains of a medieval castle on a wooded hill in the Vienna Woods near Alland, in Lower Austria. Sections of the original walls and a round tower are still standing on the elevated site.

Arnstein Castle was first mentioned in writing in 1170; Wichard von Arnstein, lord of the Hohe Lindkogel and the Peilstein, is named as its builder. His family held it until the 14th century, after which owners changed repeatedly from 1329. It was most likely destroyed during the First Ottoman Siege of Vienna in 1529 and was never rebuilt; in the 19th century stones were removed and used in the church at Raisenmarkt.

The site is protected as a heritage monument, and the surviving walls and tower give a sense of how the original structure was arranged. Visitors can walk close to the stonework and observe the building techniques used in medieval construction.

The ruins can be reached by foot along forest trails that also connect to other walking routes in the region. The ground around the site is uneven and can be slippery after rain, so sturdy footwear is a good idea.

Below the ruin lies the Arnsteinhöhle, a 128-metre cave probably used by the castle's inhabitants as a cattle stall; masonry remains at its entrance. Late 19th-century finds of cave bear, cave hyena and reindeer bones sparked a rumour of a gold treasure, and locals dug the cave and the castle rock in search of it.