Burgruine Kaja, Medieval castle ruins in Hardegg, Austria

Burgruine Kaja is a ruined hilltop castle north of Merkersdorf, above the Kajabach stream in the Thayatal National Park. Its walls stand on elevated ground, and the palace and castle chapel were restored after 1969. The site is privately owned by the Waldstein-Wartenberg family and open to visitors from May to October.

The fortress was built during the medieval period as a defensive installation to control the surrounding territories in Lower Austria. Over the centuries, it gradually lost its military importance and eventually fell into ruin, becoming the landscape feature visible today.

The castle gave its name to the noble family of Chiowe, later von Kaja, who held it from the 12th century. In the second half of the 20th century part of the ruin was turned into a castle museum, and since 1969 an association has worked to save the site, restoring the palace and the castle chapel.

The ruin and its castle museum are privately owned by the Waldstein-Wartenberg family and open to visitors from May to October. The site lies in the Thayatal National Park, 3.5 km southeast of Hardegg, above the settlement of Merkersdorf.

The ruin stands on elevated ground and offers views across the Austrian countryside and the valley below. This outlook explains why the fortress held such strategic value for controlling the region in its time.