The cellar of the Old Counts’ Mansion houses the 400-square-meter Roman lapidarium – one of the greatest lapidary collections in Slovenia.

Inscriptions and reliefs, made from Pohorje marble, bear witness to the affluence of the citizens of Celeia. The marble creations display Celtic and Roman costumes, the deities they worshipped (including Oriental divinities), and information about the organization of the town’s administration and about various crafting associations.

The outdoor lapidary collection consists of architectural remains of public and private buildings, and a rich collection of milestones which stood along the roads in the environs of Celje.