Burgruine Arnulfsfeste, Medieval castle ruin in Moosburg, Austria

Burgruine Arnulfsfeste is a fortified stronghold near Moosburg in Carinthia, Austria, with ruins positioned on elevated terrain commanding the surrounding landscape.The stone structures display typical medieval defensive architecture spanning multiple levels and sections.

The stronghold emerged in the early medieval period and was first documented in the 9th century as a control point for regional trade networks.It evolved into a strategic military position whose importance continued through the closing centuries of the Middle Ages.

Locals call the three-hill complex "Mosaburch", a name recorded in popular usage. The Arnulfhügel takes its name from the Arnulfturm, the 13th-century seat of a burgrave, a tower with two-metre walls and a twelve-metre square plan. The separate Rauthügel holds the Carolingian "heribergium", the oldest fortification of the group.

Access to the site is prohibited by the landowner, so the ruins can only be viewed from outside; do not enter the grounds. The complex lies northwest of the Moosburger Mitterteich and consists of three hills: the Rauthügel in the north, the Thurnerhügel in the southwest and the Arnulfhügel in the southeast.

The Arnulfturm on the Arnulfhügel has walls 2.1 to 2.25 metres thick around a square plan of about 11.4 metres per side, and its west side still stands 12.65 metres high. The Karolingerverein had the keep re-measured in 2002 after renovation. Access to the whole site is forbidden by the landowner.