Tower of Skulls

The Skull Tower (Cele Kula) was erected by the Turks at the beginning of the 19th century just after the Battle of Cegar, where Serbian insurgents suffered a defeat against the Ottoman rulers. The Ottomans maintained control of the area and the commander of the forces ordered his soldiers to decapitate the heads of the dead Serbs, skin them and bring them back to Istanbul to show the Sultan. In Istanbul, the Sultan order the construction of Cele Kula, a tower into which the skulls of the Serbian war dead were mortared, as a reminder of the consequences of revolt against Ottoman rule. To this day you can still see wisps of hair on some of the skulls. The actual tower is inside a larger structure to protect it from the elements. Cele Kula was declared a Monument of Culture of Exceptional Importance in 1979, and is protected by the Republic of Serbia.

Info from here