Molivos' castle is the second largest and most important castle of Lesvos. In its current form it is largely the work of the Genoese rulers with many later additions and repairs during Ottoman rule but also with modern repairs that are difficult to distinguish as they have been made with the same materials. On the hill of the castle was the acropolis of the ancient city, at least since the 5th century BC of which remains have been identified under two towers. Nothing remains from the acropolis of the early Christian and Byzantine times. The tank located in the middle of the castle courtyard is probably attributed to the Byzantine phase.

It was founded during Byzantine times on the ruins of the ancient fortification, mainly on the southern side. According to one version, it was founded in the 6th century under the emperor Justinian.
In 1128 it was conquered by the Venetians and from 1204 to 1287 it was under the possession of Baldwin II of Flanders. According to another view, the castle was built after the middle of the 13th century in order to restrain and fight back against the Turkish and Frankish raiders.
What is certain is that at the end of the 13th century the castle passed into the hands of the Genoese, who received it completely destroyed. In 1373 Gatelouzos Francis I brought reinforcements and attempted to repair the castle. Its current form is the result of the works and repairs during the 14th century and the Ottoman additions after 1462.

The castle of Mithymna (ancient name of Molivos) in its present form consists of 10 towers with corresponding bastions, 2 bastions connected to a curtain wall, the "acropolis" (citadel) in its northeastern part, five buildings inside the castle (two of which are in ruins) and two parapets to reinforce vulnerable points.

The castle has the shape of an irregular trapezium, with a side of 70 m. Most of it is built according to the pseudo-isodomic masonry system. Large carved stones of basalt rock were used. The NE side presents smoother terrain, which necessitated the construction of higher ramparts to ensure greater protection.

Access to the castle is through three successive gates. The outer gate opens at the southernmost end of the outer enclosure and dates to the Ottoman period based on the inscription it bears and the configuration with a pointed arch. A short distance away there is a second entrance with a vaulted passageway with transverse ribs, leading to an open air area protected by the walls. Going up the cobblestones the third gate in a row appears, which is the main entrance to the castle and dates back to the 14th century. The wooden, metal-clad door that closes the entrance (from the Ottoman period) is of great interest. Inside the castle, the surviving buildings date back to the Ottoman period.

The interior of the castle is divided into several levels. A large underground water tank with an arched roof survives from the Byzantine era. It provided drinking water to the besieged. Since the time of Ottoman rule, a three-part building with a dome has been preserved in the southern part of the entrance. It was used as a prison, while according to another version it was used as a storehouse for gunpowder. In the eastern part of the main entrance there is a building, also from the Ottoman period, consisting of four rooms. In the northern part of the fortress, two more buildings were probably the garrison's quarters. Also, a drainage pipe shows that the area was probably used for sanitary reasons. In many parts of the walls the visitor will notice built-in plaques bearing inscriptions or other insignia.
Restoration work began in 1976 and until 1981 fixings, restorations and joints were made in parts of the walls. In 1993, a breach in the southern rampart was repaired.
In addition, the Castle of Mithymna joined the "Castles Peregrination" program. In this context, extensive repairs and restorations were made and it was inaugurated on October 3, 2008.