The Fortress Marienberg lies 100 m above the river Main. For nearly five centuries the Marienfeste was the palace of the prince-bishop of Würzburg. The first fortress on the site was a Celtic refuge castle (ca. 1000 BC). A medieval fortress was built here. In 1631, during the Thirty Years' War the fortress was taken by the Swedish king Gustav Adolf. After the year 1642 the fortress was rebuilt. The palace was remodelled in Baroque style. Two rings of heavy fortifications were added. The Castle keep - within the inner court - is an old part of the fortress. It was built in the 13th century. It served as a prison for many centuries.