Mithymna Castle, Medieval fortress in Mithymna, Greece.

Mithymna Castle is a fortress on a hill above the village of Molyvos, with a roughly quadrilateral plan measuring 70–80 metres per side and covering about 5 stremmata. Ten square or round towers linked by curtain walls enclose the summit, and the keep stands at the highest point. The walls are built of red-brown basalt and grey trachyte from the hill itself, and the medieval name Molyvos comes from the lead-grey colour of the trachyte.

The hill was the acropolis of ancient Methymna, and some remains of those fortifications survive in its southern part. A castle existed by the Byzantine period, possibly built under Justinian in the 6th century; the Venetians held the site from 1128 and built a new castle. The Genoese took it in the late 13th century, and the present structure was rebuilt in 1373. After 1462 the Ottomans made repairs and additions, which ceased by 1750.

The fortress displays architectural marks from Byzantine, Genoese, and Ottoman periods that reflect the changing powers across this island. Visitors can see these different building styles mixed throughout the walls and towers that accumulated over time.

The castle is accessible from the old town of Molyvos via an uphill path that is not very long but does require climbing. Visitors should wear sturdy shoes and allow time for the ascent, especially during warmer weather.

Local legend claims that stones from an earlier fortification were built into the walls of the present castle, supposedly coming from a site connected to ancient conflicts. These possibly reused ancient components literally link different ages together in one location.