Sonnenberg, Mountain summit in Hornstein, Austria.

Sonnenberg is a mountain in the Leitha range near Hornstein that rises to 484 meters (1,588 feet) and contains multiple forest trails winding through its slopes. The terrain is covered with trees that create natural corridors for walking.

The 15-metre steel tower on the summit was built as a Cold War border watchtower and moved to this spot after the Iron Curtain fell; it now serves as a lookout. The mountain lies on the Leitha range ridge along the border between Lower Austria and Burgenland, and its 484-metre summit is the highest point of that ridge.

The name Sonnenberg ("sun mountain") comes from its sun-exposed southern slopes above Hornstein. The Hungarian name Naphegy carries the same meaning. Since 2011 the standard hiking tip in Der Standard has drawn walkers up the old forest road from Hornstein, past the Gloriette above Eisenstadt, to the summit cross and tower.

A circular route of about 8 to 9 kilometers starts near Hornstein cemetery and returns to the same point, taking between two to two and a half hours to walk. Sturdy shoes help on steeper sections where roots and loose ground appear.

The 15-metre steel tower on the summit, the Sonnenbergwarte, was built as a Cold War border watchtower and moved here after the Iron Curtain fell; it can now be climbed as an observation tower. On clear days the view reaches as far as the Wechsel.