Bač Fortress Bač Fortress ( Serbian : Бачка тврђава / " Bačka tvrđava " ) is the medieval fortress in Serbia , Vojvodina . It is located in the town of Bač . Background . Medieval fortress in Bač is known as one of the great fortress of its time in Pannonian plain . It was built by the town of the same name to the west , on a small island on the river Mostonga . The Fortress in Bač is known as one of the so - called " Water Towns " , because it used to be surrounded by the Mostonga river on all sides , approachable only by the drawbridge s . The fortress has a pentagonal basis on which corners are projecting towers . Inside the fort there is another , superior , central ( or dungeon defender ) tower . The tower has served as an important part of the defense system of the fortress , but also as an observation post . Earliest Times . According to some chronicles , Bač was also an Avar fortress ( 873 ) . It was inhabited by both , Avars and Slavs . The Hungarian King Stephen I founded the Bač County , and at the turn of the first millennium Bač was already a well built and fortified town , connected to the Danube by the river Mostonga . During The Árpád dynasty , Bač became not only the ecclesiastical seat of the Archdiocese , but a royal city , where nobles and rulers came to their councils and assemblies . In the twelfth century Bač fortress is first mentioned in written sources . At the head of Bačka county there were governors appointed by the king . Until the Mongol invasion in 1241 , the governors were the managers of the area . They collected donations , set up military and civilian commanders . In addition to the governors of the fort there were also lower prefects , judges , and military personnel . Since the twelfth century , the custom of annual royal visits to the fort was established . Remodeled . At the beginning of the fourteenth century a dynasty change occurs in the Kingdom of Hungary . The Hungarian King Charles Robert I ( 1310 – 1342 ) founds the new town ( fortress ) on Mostonga between the 1338 and 1342 , with the quadrangular tower with six floors , important for the defense of the city . There primarily existed eight towers , various chambers of the county manager , a chapel , guard towers , kitchen , well , barn , and various other premises , the remains of which stand today . Ottomans . In the aftermath of the Battle of Mohács from 1526 , the city and fort became an Ottoman possession . Dating from this period are also the descriptions of Bač Fortress by Evliya Çelebi , when Bač became a part of the Ottoman Sanjak of Segedin . According to the traveler description , the city was surrounded by a wide circle of trenches filled with water . Surrounding the city was also a developing civilian settlement , which could be entered through the fortress drawbridge gate , named " The Spike " . Last Years . In the time of Rákóczi Rebellion ( 1703 – 1711 ) the fortress was burned , destroyed and abandoned . However , Bač fort still remains the best preserved medieval fortress in Vojvodina . The ruins of the fortress in its present state , consist of a base in the form of an irregular pentagon , four lateral , and one central tower 18 meters high , which is partially reconstructed . In its history the fort underwent various renovation and demolition , and the remains still present a challenge for scientists and tourists alike .