Ad Pirum, Roman fortification in Hrušica Plateau, Slovenia.

Ad Pirum is a Roman fortification on the Hrušica Plateau featuring oval-shaped ramparts with substantial stonework typical of frontier defenses. The remains show the layout of barracks, watchtowers, and enclosed compounds where soldiers lived and worked.

Ad Pirum was built in the 320s, during the late Roman Empire, on the Hrušica Plateau. It was the central fort of the Claustra Alpium Iuliarum, a network of forts and walls securing Roman Italy from eastern invasions. It also guarded the road between Italy and Pannonia, and the Via Gemina linking Aquileia and Aemona.

The name Ad Pirum refers to a pear tree that stood near the pass, Latin pirum meaning pear. The fortress guarded the Via Gemina, the road linking Aquileia with Aemona, today's Ljubljana; the route over the Piro pass was the shortest between Castra ad fluvium frigidum and Emona.

The site sits at high elevation in a mountain setting and requires sturdy footwear and basic fitness to explore the terrain properly. Information panels at the location help visitors understand what they are seeing among the scattered ruins.

The fortress could shelter up to 100,000 soldiers in wartime, though its usual garrison was 500 men. It stood on the shortest route between Castra ad fluvium frigidum and Emona, a detail that made the Piro pass the preferred crossing.