In the courtyard of the Archaeological Museum of Thiva today stands the medieval tower of Saint Omer which is considered to have been built at the end of the 13th century by the lord Nikolaos II St. Omer (1258-1294), to whose family half of the city of Thiva was granted as a dowry by the rulers of the Duchy of Athens.

It is the best preserved part of the medieval fortification of Thiva, it is of rectangular shape and its height reaches 14 meters. It originally had at least three floors, of which the first two are preserved today. For protection reasons, the entrance was on the first floor and access to it and to the other levels was by wooden stairs.

The tower in the late 19th century was used as a prison. On the walls of the ground floor there are drawings of the prisoners, where they recorded their unknown to us remaining sentences.

The recent work of fixing and restoration of the monument brought to light important archaeological evidence, while a short report on the operation of the building, as well as the scattered towers throughout the countryside of Viotia, landmarks of the region until today, was prepared. The tower is open to visitors.