Mildenstein Castle, Medieval castle and museum in Leisnig, Germany.

Mildenstein Castle stands on a rocky spur above the Freiberger Mulde, its long battlemented curtain wall following the ridge. The inner courtyard holds a 32-metre bergfried whose lower eight metres are laid in local quartzite blocks and whose upper part is brick; a second bergfried rises in the outer ward. The chapel, rebuilt around 1400, and the late 14th-century granary, Herrenhaus and Pagenhaus cluster behind the gatehouse.

Mildenstein Castle was probably built in the 10th century and was first mentioned in 1046, when Emperor Heinrich III gifted the Burgwards of Colditz, Rochlitz, and Leisnig to Agnes of Poitou. In 1084 Heinrich IV enfeoffed it to Wiprecht of Groitzsch, and in 1148 Duke Friedrich III of Swabia, later Emperor Barbarossa, acquired it and made it an imperial property in 1158. The Burgraves of Leisnig, enfeoffed with the castle, lost it in 1365 to the Margraves of Meissen; under Margrave Wilhelm I it was rebuilt in the late 14th century.

The castle chapel was built around 1100 and is the oldest surviving building on the site. Around 1400 to 1420 it was extended with a Gothic choir. The name Mildenstein only came into use in the late 14th century, taken from an earlier castle destroyed in 1232; until then the castle was called Burg Leisnig.

The 32 m bergfried in the inner courtyard is climbable as an observation tower, with a view over Leisnig and the Mulde valley. The museum's permanent exhibitions cover Christian art, castle life, early modern administration and the prison system; guided tours, concerts and reenactment events are offered. The grounds lie at the northern edge of Leisnig, opposite Fischendorf.

The castle's second bergfried, in the outer ward, survives as a ruin in the town below the main complex and is not part of the museum. Two bergfrieds on one castle site are rare in Saxony; besides Mildenstein, only Rochlitz, Kohren and Strehla castles are known to have had two.