Schlössl Rosenau, Medieval castle ruins in Sonntagberg, Austria

Burgruine Gleiß is the ruin of a hilltop castle on a conglomerate rock on the right bank of the Ybbs in the village of Gleiß, part of Sonntagberg municipality, 2.4 km northwest of Sonntagberg. It was burned by French troops in 1806 and left to decay, so only masonry remains of the former fortress stand today.

The fortress was established around 993 as one of the earliest fortified structures in this Lower Austrian region. It underwent several rebuilding phases over time before eventually falling into ruin.

The name Gleiß derives from Gluzengisazi, 'seat of Gluzo', after the Slavic settler Gluzo who began clearing land here; the site appears in a 993 deed of Otto III. Epitaphs of the Strasser family and Anna Magdalena Geyer of Osterburg, granddaughter of Wolfgang Strasser, are kept in the parish church of Opponitz, once part of the Gleiß estate.

The ruin stands on a conglomerate rock on the right bank of the Ybbs, 2.4 km northwest of Sonntagberg's main village in the Gleiß district. It is a protected monument; the site is unbuilt and open, so expect uneven ground on the rock. No visitor facilities are documented.

The name Gleiß derives from Gluzengisazi, "seat of Gluzo", after the Slavic settler Gluzo who began clearing land there; a 993 charter of the future Emperor Otto III is the oldest written record of settlement in the old March of Ostarrîchi. The copperplate engraving by Matthäus Merian from around 1649 shows the castle before its destruction.