Pythion Castle is built on a low hill at the western limits of the ​​Evros River riparian zone, at a distance of 15 km from both Didymoteichoand Adrianople . Founded between 1330 and 1340 by John VI Catacuzenus, it served as a military fortress and personal refuge during the civil wars of the 14th century. After the castle fell to the Ottomans, a new settlement called Kaleli Burgas grew up around it. The village is the burial place of Patriarch Cyril VI, who was hanged during the revolution.

The fortifications included a large external wall, parts of which still stand among the modern homes, and a smaller internal enceinte. Both enclosures are reinforced by towers on the outside, as well as by two large, square towers of different sizes flanking the entrance to the interior of the castle, at the point where the two walls join.

The largest tower stands three storeys tall, and has survived intact to a height of 17 metres. The floors are roofed over by four hemispherical domes, and are linked via a staircase built inside the east wall. On the south side of the upper floor a small exit led to the ramparts of the median wall. Surviving sections indicate that there were originally two further floors, the first of which projected and had machicolations for defence. The tower most probably served as a residence for the castle owner, particularly on the third floor, where there was a fireplace and a cupboard.

The smaller tower is almost square, slightly oblique and stands at a height of 20 metres, comprising four independent, single-roomed storeys with domed vaults. Evidence points to the existence of a fifth floor, though this has not survived. The ground floor is devoid of windows and was probably destined for use as a prison. An opening in the floor of the second level gave access to the ground floor via a wooden, probably portable ladder; the other floors were not linked, each having independent access. This tower was exclusively intended for military use.

Pithion Castle combines features of western forts, which were the headquarters of feudal lords,  with defensive castles  aimed at controlling and protecting the surrounding  area. It is a pioneering work in Byzantine fortifications, and one which continues to impress visitors to this day,despite the fact that only a small part of its original bulk survives.