The Thracian Cult Complex near Starosel centres on the large Chetinyova Mogila tumulus, with tomb and temple remains dating to the 4th or 5th century BCE. It is one of the strongest ancient sites around Hisarya and adds a pre-Roman layer to the area's better-known baths and walls.
The site is outside town, so plan transport rather than assuming it is a casual stroll. The reward is a compact, atmospheric look at Thracian ritual architecture in the hills.