[1][2] The fortress was controlled by the Byzantine and the Bulgarian Empires until the region was conquered by the Ottoman Turks in the second half of the 14th century.

[1][2][4] It rises on an elongated, rocky, steep peak with very good visibility, covering a long distance in the surrounding area.

[1][3][4] The fortress was studied in 1971–1973 by archaeologist Stamen Mihaylov and historian Ivan Balkanski, then curator of the Kardzhali Museum of History.

More than 1,000 objects were found during the excavations, including iron arrow tips, bronze and silver crosses, copper coins, clay vessels.

[3][9] A research in 2006 found traces of large scale fortification works on the ridge descending in direction northeast from the promontory, where Ustra is situated.

[2][3] The fortress was mentioned by name in the works of two 13th century Byzantine chroniclers, George Akropolites and Theodore Skoutariotes.

[9] They both recorded the 1254 successful campaign in the region by Bulgarian emperor Michael II Asen, following the recent loss of the lands in the Rhodope Mountains to the Nicaean Empire.

Akropolites wrote: "He [Michael II Asen] descended from the Haemus and when he crossed the Evros, in a short time subjugated huge territory and took many towns without an effort.