Opensten, Medieval castle in Södra Åsarp, Sweden

Opensten is a medieval castle in Tranemo Municipality, built on elevated ground with stone foundations and displaying traditional Swedish fortification features. The structure stands as a physical example of defensive architecture from the Middle Ages.

Opensten is first documented in 1355, when Duke Eric II of Saxe-Lauenburg was its lord, though it may date from the 13th century as a guard on the route along the Ätran. A medieval note records its rebuilding in 1362 alongside Axevalla hus. It passed to Valdemar Atterdag's troops in 1366, to Bo Jonsson Grip in 1383 and by his will to Queen Margaret in 1388; Engelbrekt Engelbrektsson burned it in 1434.

The name Opensten appears in medieval records also as Apensten, Oppensten and Opänsten, and earlier as Husberget. The hill was long called Husberget before the castle name took over. The site is registered as Södra Åsarp 15:1 in Fornminnesregistret, and the county museum's records describe the ruin as a medieval castle site.

The site sits on elevated ground and can be reached by following local roads, with the raised position offering good views of the surrounding area. Visitors should be prepared for uneven terrain since the structure stands on natural ground without modern modifications.

The site is listed in the Fornminnesregistret, Sweden's heritage register, with a dual classification as both castle and monument. This dual designation reflects its importance as both an architectural and archaeological record of the medieval region.