Crowning the hill is Marksburg Castle, its unusually slender keep towering above the surrounding courts and ramparts. Beginning with the construction of the keep in the 12th century, the castle grew into its present shape throughout succeeding centuries. In the Middle Ages, the strengthening of the castle's defences became essential and urgent by improvements made to firearms. Huge outworks date from this time, as does the conversion of a gateway in the outer wall to a strong bastion. It is mainly due to these extra defence works that the castle was never seriously attacked. In more peaceful times, it was used mainly as a state prison.