Petrich Kale (Petrich Hill) was an important fortress in and a key element of the defense system of Bulgaria in the Middle Ages. Four kilometers to the north of Avren Village and to the East of the Razdelna, a small station on the Sofia-Varna raliway track, the fortress was originally built in the 5^th or 6^th century AD to be leveled with the ground several centuries later and re-built sometime in the 11^th or 12^th century AD.  During the third crusade of the united Christian armies the fortress was conquered and destroyed on 7^th November 1444 by the Polish-Hungarian King Wladislaw III Jagelo.

The rocks nearby offer excellent conditions for mountain climbing.