Žrnov (Serbian Cyrillic: Жрнов) or Žrnovan (Жрнован) was a medieval fortress on the highest top of the Avala Mountain, at 511 metres (1,677 ft), in Belgrade, Serbia.

Archeologist Miloje Vasić believed that the vast mines of cinnabarite (mercury-sulfide) on Avala were crucial for the development of the Vinča culture, on the banks of the Danube circa 5700 BC.

[1] The first miners on Avala recorded in history were the Celtic tribe of Scordisci and it is believed that they built the first outpost on the top of the mountain.

[2][3] The top of Avala proved to be suitable for building, so the Romans built a fortified outpost, probably on the foundations of the older Celtic one.

[3] Apart from guarding and controlling the access roads to Singidunum, predecessor of Belgrade, the outpost was also important for the protection of the mines on the mountain, which were exploited by the Romans.

The Ottomans conquered the fort in 1442 and further fortified and strengthened the eastern and southern ramparts, on the orders of the Hadım Şehabeddin, a eunuch beylerbey of Rumelia Eyalet.

[6] By the Peace of Szeged, the Serbian despotate was restored and the Ottomans had to withdraw from the fortified towns: Golubac, Kruševac, Novo Brdo and Žrnov.

With expansion of the walls and ramparts and the building of the dry moat, they effectively turned it into the starting point of the constant harassment of the Hungarian defenders of Belgrade.

The Ottomans began calling the fort havala, which means obstacle, and that gave the name to the entire Avala mountain.

[4] One of the surviving Serbian sources say that "from the ruins of the medieval little town of Žrnov, which was named Avala, the Turks built a proper marauders' tower, roofed with lead.

[3] On the night of 13/14 October 1915, the combined squad of Belgrade's Defense held the line of Avala-Zuce, against the joint Austro-Hungarian and German offensive.

Austro-Hungarian 9th Hill brigade of the 59th Division took the front rim of Avala on 16 October, with an assignment to push Serbian forces from the mountain.

On the same day, the command of the Belgrade's Defense ordered a retreat to the new positions so on 17 October occupational forces reached the southern section of Avala, conquering the entire mountain.

In the valley, below Žrnov, on the grave of one of them, they placed a wooden cross with the inscription in German: "One unknown Serbian soldier".

[3] The rampart which encircled the Lower Town, effectively forming it, began at the fortified tower on the southwest, making a semi-circled turn to the south where it additionally extended to the southeast following the slope.

Parts of the grenade are found under the skull, almost as a pillow, while the skeleton had the blown left side of the chest, so it is estimated that he was killed by an Austro-Hungarian howitzer while he was watching from the lookout.

On the eastern side of the pedestal a plate was placed with the inscription: "To the fallen heroes in the wars for liberation and unification 1912-1918, this monument is erected by the thankful people of the Vračar District.

On the western side of the horizontal arm of the cross another inscription read: "Unknown Serbian soldier confirmed by the commission on 29 November 1921".

The old monument was completely demolished, except for the six-armed cross which was moved to the churchyard of the Church of Saint Mary Magdalene in Beli Potok, on the foothills of Avala, where it is still located.

[12] The remains of the fortifications were demolished by dynamite in 1934,[13] in order to clear the site for the construction of the new monument, that is, an entire complex of the mausoleum.

[4] The king was assassinated in Marseilles on 9 October 1934, three and a half months after he laid the foundation stone for the new monument, which was finished on 28 June 1938.