Burg Werfenstein, Medieval castle ruin in Sankt Nikola an der Donau, Austria

Burg Werfenstein is a lowland castle ruin on the Danube at Struden, in the municipality of St. Nikola an der Donau. Parts of the ring wall and the tower survive; the tower's corner is built of layered masonry, the rest of rubble stone, and the west and south sides were probably added in the late Gothic period. The old entrance door sits seven metres above the courtyard, and the site covers 716 m².

Werfenstein was first documented around 1256. In 1294 Duke Albrecht II besieged it for three weeks together with Freyenstein and Pain. The Prueschenk family moved to the new Greinburg at the end of the 15th century, leaving Werfenstein roofless by 1531; it burned in 1645 and the Palas was demolished in 1780.

The name Werfenstein comes from Old High German hwërbo, meaning whirlpool or eddy, so it reads as "rock by the Danube whirlpool". A Spanish account instead ties the name to stones thrown in defence during a siege. The castle formed part of a chain of toll and guard posts on the Danube, and with Wörth on the opposite island it could stretch chains across the river to block shipping.

The ruins are visible from the Danube cycling path, which has passed nearby since 2010 and provides good views of the site. The location is easily accessible from the path for photos and observation.

A doctor from Linz bought the ruins in 1963 and transformed them into a private home by blending modern structures with the medieval walls. This unusual restoration shows how historical sites can be adapted for new purposes.