In 1809, the Ottoman Empire wanted to warn the Serbian people not to attempt any more foolish rebellion*, so it created this macabre monument:
Serbian forces dug new trenches under the local command of the Duke, Stevan Sindjelic. Since there was such a great defensive strategy
between the Serbian leaders, Sinjelic found himself unprepared. He realized the situation was hopeless, so he blew up the ammunition store with his own pistol.
Afterwards, Turkish commander, Hurshid Pasha, ordered the decapitation of all the Serbian bodies, even erecting a monument using the heads as building material. The tower itself was 10 feet high, contained 952 skulls, and was topped with the head of Sindjalic himself.
During the later part of the 19th century, the skulls were removed, both as macabre souvenirs of battle and proper burials. In 1892, there were only 50 left on the tower and a chapel was built over top to preserve what little remained.
Link | More at Atlas Obscura and Wikipedia (Photo: Wikimedia/Freerspska.org)
*note: the warning didn't exactly work. A few years later in 1815, Serbia rebelled again and that time, achieved semi- and then full-independence.