At one of the northern hills of Sakar Mountain, about 2 kilometers from the village of Glavan (Galabovo municipality) are located remains of fortress walls, which are supposed to be the stronghold of Balzena. In the science this fortress was called also "Kaleto" or "Gradishteto" while the local call it "Balzena".
The first archaeological research of fortress of Glavan date back to the XIX century, when the brothers Shkorpil make its first plan (they are no so accurate is turned out later) and define it as a Roman or medieval construction. They suggest that here was situated the capital of the Celts - Tilis, which name they associated with the known Tuleus castle from the description of Procopius De aedeficiis.
According to other scholars (Vassil Mihov and I.Velkov) there was a large Thracian fortress which was used in VII-XII century too.
From the inscription on the Syuleymankyoyskiya column, that was found near the village of Sechishte (the previous name of the village was Syuleymankyoy), is suggested the castle is one of the border fortresses between Bulgaria and Byzantium. The inscription define the terms of a peace agreement between the parties and are set the the borders between them in the regions of Strandja and Sakar. Among the border fortresses is mentioned the names of less or more accurately localized medieval castles - Debelt, Peramokastel, Balzena, Agatoniki, Constantia, Macri Livada.
Because the names of the fortresses of Balzena and Agatoniki can not be found in earlier written sources it is suggested they were built probably after the permanent placement of the Slavs on the Balkans and the formation of the Bulgarian state.