Burg Mellnau, Medieval castle ruins in Wetter, Germany.

Burg Mellnau is a medieval castle ruin featuring a circular tower roughly 9.5 meters in diameter with surviving ring walls forming an oval shape around the complex. The tower can be reached by climbing 89 steps, from where visitors gain views across the surrounding landscape.

Mellnau Castle was first documented in 1263 as "Elenhouch". Archbishop Siegfried III of Mainz probably built it around 1248 to replace the destroyed Hollende Castle and protect the Stift Wetter. In 1301 Ulrich I of Hanau captured it during the Rhenish customs war; in 1464 it was pledged to Hesse and lost military importance. From 1510 it was no longer occupied and decayed.

The castle gave its name to Mellnau only around 1500; before that it was recorded as Elnhouch and Elnhoch. The name Mellnau was transferred from the settlement below, which grew into the village of Mellnau, today a district of Wetter.

To enter the tower, visitors must obtain a key from the nearby Kuckuckshütte before climbing. The ascent requires some physical fitness as the steps are steep and the opening at the top is narrow.

The original gate of the castle was walled up; it stood directly east of the bergfried, while today's entrance breaks the ring wall in the southwest. The round tower, built around 1329, stands free in the eastern part of the core castle and is used as an observation tower.