Altrasen Castle, Medieval castle in Niederrasen, South Tyrol, Italy.

Altrasen Castle, also called Castelvecchio Rasun, is a ruined hilltop castle in a wood above Niederrasen in the Antholz Valley. What remains are parts of the keep's walls and the lower storeys of the residential palace, the most substantial surviving section, plus remnants of a square tower, ring wall and tower courtyard from the 16th century.

The fortress was built during the Middle Ages to monitor and protect traffic moving through the Antholz Valley as a strategic stronghold. Over time it lost military importance, was eventually abandoned, and deteriorated into the ruins visible today.

The name Altrasen distinguishes the castle from Neurasen (Castelnuovo Rasun), built around 1230 by Ulrich von Rasen on the other side of the valley after the lords of Rasen lost the old castle to the counts of Tyrol; the resulting feud between the two branches of the family is recorded. The site later gave its name to the Gericht Altrasen, the district court documented in 1484, whose jurisdiction reached beyond the Rienz as far as Olang.

Access is through marked hiking trails from Niederrasen village that are easy to follow from there. Sturdy footwear suitable for mountain terrain is recommended, and the local tourism office can provide information about guided visits.

The ruins provide views across wide mountain ranges and the valley floor below, a panorama that was once important for defenders. This outlook helps visitors today understand the strategic positioning and purpose of the fortress.