Schloss Illmau, Medieval water castle in Kautzen, Austria.

Schloss Illmau is a medieval water castle in Kautzen featuring a two-story three-winged main structure with a four-story tower on the southwest side. The building displays late Gothic elements especially at the gate and was built with water defenses.

The castle was first documented in 1337 under the Hauser family's control. It passed to the von Herberstein Grünne family in 1390, beginning a long period of their ownership.

The name Illmau, recorded in 1337, means "elm meadow" (Ulmenau); earlier spellings include Ilmenau, Hymlau and Ylmau. From the Middle Ages until 1848 the castle was the seat of the Illmau estate, which held local political administration, civil justice, tax collection and poor relief over its subjects. Since the 17th century it was used as a farm building and granary, which caused its decay.

The castle sits about 600 meters northwest of Kautzen's parish church and is visible from outside. Since it remains privately owned, interior visits are not possible, but the architecture and water defenses can be viewed from the exterior.

The castle marked a strategic point at the historical border between Austria, Bohemia, and Moravia, highlighting its importance as a frontier fortress. Its location made it a key fortification in a network of defensive structures across the region.