Schloss Haindorf, Protected castle in Langenlois, Austria.

Schloss Haindorf is a three-story castle in Langenlois with a white facade, symmetrical windows, and a central entrance framed by stone columns. The grounds extend across gardens and open spaces, creating a substantial estate that now serves as a seminar hotel.

Schloss Haindorf grew out of the Feldmühle, first mentioned in 1624, whose roots likely reach back to the Loisbachmühle recorded in 1407. From 1680 to 1876 the counts of Grundemann owned the estate; after them Leopold Popper von Podhragy acquired it. It decayed under Baron Sales and was restored by the heraldist Friedrich Graf Lanjus von Wellenburg. Soviet occupation soldiers devastated it, forcing the count's widow, the poet Theres Lanjus, to move out in 1945.

The castle grounds host operetta performances in summer, attracting hundreds of visitors to enjoy music and theater in a garden setting. This tradition has made it a cultural meeting point for the local community.

Schloss Haindorf is run by the Lower Austrian construction guild's training centre, the BAUAkademie, as a seminar hotel and education site, so it is not a museum open to casual visitors. The summer operetta performances take place in front of the castle, with around 1,500 seats in the arena and a wine tasting set up in the castle park. The park behind the building reaches as far as the Kamp river.

The castle's name Haindorf distinguishes it from another Schloss in the same cadastral district, so it is often called the lower castle. From 1996 the Schlossfestspiele Langenlois have staged operettas in front of the building, with an arena of nearly 1,500 seats, making it one of Lower Austria's largest summer theatre venues.