Žrnov or Žrnovan (Жрнован) was a medieval fortress on the highest top of the Avala Mountain, at 511 metres (1,677 ft), in Belgrade, Serbia. The Ancient Romans had built an outpost there, and later the Serbs expanded it into a fortress. It was completely demolished in 1934 to make the way for the Monument to the Unknown Hero.

Avala is a mountain in Serbia, overlooking Belgrade. It is situated in the south-eastern corner of the city and provides a great panoramic view of Belgrade, Vojvodina and Šumadija, as the surrounding area on all sides is mostly lowlands. It stands at 511 metres (1,677 ft) above sea level, which means that it enters the locally defined mountain category just by 11 m (36 ft).