Fortress Marienberg (German: Festung Marienberg) is a prominent landmark on the Main river in Würzburg, Germany. It has been a fort since ancient times. After Gustavus Adolphus of Sweden conquered the area in 1631, the castle was reconstructed in the Baroque style. Today, it is a park and museum. In 704 A.D., the Marienkirche was built atop a former Celtic shelter and in the 13th century was surrounded by the first fortification. In 1482, the main castle was encircled by a medieval ring wall with the Scherenberg gate. Some of the parts of the fortress accessible to the public are the Scherenberg-Tor (Gate) the Burgfried (keep), a chapel, a well house, Bibra Stairs / Lorenz von Bibra apartments, and the Julius Echter Apartments.

In May 1525, during the Peasants' War (Bauernkrieg), a peasant army of 15,000 men surrounded the fortification (seat of the bishop of Würzburg) but could not penetrate the concentric walls built on a steep incline. When their leader, Florian Geyer, went to Rothenburg ob der Tauber in early June to procure the heavy guns needed to at least attempt to breach the walls, the leaderless peasant army camped out around the castle, allowed themselves to be outflanked by a professional army in the service of the bishop. More than 8,000 peasants were either slaughtered or blinded on the bishop's orders. The Nazis would lionize Florian Geyer 410 years later, as part of the National Socialists' desire to connect with the common man and turn them away from the Catholic Church.

In about 1600, Julius Echter rebuilt the fortress into a Renaissance palace. After the conquest by Gustav II Adolf of Sweden in 1631 (Thirty Years War), the fortress was reconstructed as an even more formidable baroque fortification, and a princely park was laid out.

Still, during the Napoleonic Wars, the fortress was captured and, of course, the fortress wasn't defended in 1945 when the US Army quickly captured the side of the Main river on which Marienberg is located, across from the city center of Würzburg.

The Baroque Armory, built 1702-1712, houses the Mainfränkisches Museum, an excellent collection of Franconian works of art, including world-famous sculptures by Tilman Riemenschneider. The Fürstenbau Museum in the princes’ wing of the fortress offers a stroll through 1200 years of Würzburg’s history. The mighty Fortress Marienberg is the symbol of Würzburg and served as a home of the prince-bishops for nearly five centuries.