Court of Ostein (Osteiner Hof), Mainz

The Court of Ostein (Osteiner Hof) is a Baroque-era palatial mansion located on the southern edge of Schillerplatz square in Mainz. Constructed between 1747 and 1752 by architect Johann Valentin Thomann, the building was commissioned by Franz Wolfgang Damian von Ostein, the brother of Prince-Bishop Johann Friedrich Karl von Ostein. Its facade is distinguished by three round protrusions (risalits) at the entrance and corners, as well as lavish Rococo decorations, including cartouches representing air, earth, and water. The gods Diana and Mars are depicted on the balcony doors.

Although built as a noble residence, the von Osteins did not inhabit the mansion for long. In 1798, following the French occupation of the left bank of the Rhine during the Revolutionary Wars, the mansion was repurposed as the administrative seat of the French Mont-Tonnerre département. After the Napoleonic era, the building continued to serve governmental purposes, gaining the nickname "Gouvernement" during Emperor Wilhelm I’s time as military governor of Mainz (1854-1859). It later served as military headquarters during the Franco-Prussian War.

Notably, General Hugo von Kathen announced the start of World War I from the balcony of the Osteiner Hof in 1914. The mansion was severely damaged by fire during World War II but was restored in 1947-1948. From 1958 to 2014, the Bundeswehr used it as a military headquarters.

The Osteiner Hof is also central to Mainz’s carnival traditions. Every year, the start of the carnival season is proclaimed from its balcony on November 11 at precisely 11:11 AM.