 As is the case with many fortresses , Golubac 's structure was modified over time . For years , there were only five towers . Later , four more were added . The towers were all built as squares , a sign of the fortress ' age , showing that battles were still fought with cold steel . Once firearms came into use , the Turks fortified the western towers with cannon ports and polygonal or cylindrical reinforcements up to two metres thick . After the Hungarian raid in 1481 , they added the final tower , complete with cannon embrasures and galleries . 
