On the occasion of its 120th anniversary in 1982, the then Kur- und Verkehrsverein donated a replica of the Heddernheim Mithras stone, the original of which is in the Nassau Antiquities Collection, and gave it to the town. The stone was placed in the immediate vicinity of the site of a Mithras sanctuary, which was located near the Heidenmauer below Coulinstraße at the foot of the Römertor and was discovered during road construction. Over time, this developed into a small Roman open-air museum on the site of the former cemetery.

The street Am Römertor, which was originally laid out as Kirchhofgässchen and leads in a steep curve from the city center to Coulinstraße, has served as access to the copies of Roman stone monuments found in Wiesbaden ever since. The Roman Open-Air Museum displays, for example, casts of soldiers' gravestones from the Chatten War of 83-86 AD, an inscription referring to the restoration of a Jupiter Dolichenus temple in 194 AD or the building inscription for an assembly building of Roman merchants in Aquae Mattiacae from 212 AD. The originals can be found in the Nassau Antiquities Collection.