From our Serbian travel guide - 52 Weekends in Serbia - that I picked up at the DMWC event with the Serbian Ambassador (see this blog post DMWC event with Serbian Ambassador):
"The Serbian insurrection army conquered the Hill of Cegar in 1809. It was led by Commander Duke Stevan Sindjelic. Having realised that he was unable to defend from the more numerous Turkish army, he decided to act heroically: he lured them to a position, then blasted them all. When the commander of the city learned about the event, he ordered to have the killed Serbs decapitated and their heads built into the tower. The Skull Tower, i.e., Cele Kula, is a unique monument of brutal revenge."
Originally the tower included 952 skulls on 4 sides in 14 rows; 54 skulls remain embedded in the walls today.
See this "Atlas Obsucra" article for more/photos:
Interestingly, "Mental Floss" lists Cele Kula as the top building made with bones:
|the chapel built around the Cele Kula|
|detail on the chapel that was built around the monument to preserve it|
|(photo from the website timetravelturtle)|
|this is believed to be the skull of Commander Sindjelic|
|(photo from the website davidsbeenthere)|
|the Visitor Center/ticket booth|