Einhard, Charlemagne's biographer, mentions his two visits to Wiesbaden in 830: "castrum, quod moderno tempore Wisibada vocatur" ("The castrum that is called Wiesbaden in more recent times."). Opinions differ as to the meaning of "castrum". Some researchers believe that it refers to the royal court, "regia curtis", whereas Otto Renkhoff assumes that Einhard intended to use this term to characterize the late Roman fortifications, in particular the Heidenmauer. In his opinion, however, the "curtis", first mentioned in 1123 but dating back to the 9th century, should be referred to as a royal court.

The royal courts were royal farmsteads introduced by the Carolingians for the administration of their dominions. They were used to develop and expand the land as well as to provide food and accommodation for the king and his entourage.

According to Renkhoff, the complex with outbuildings and gardens could have extended from the middle of Saalgasse southwards over the Kaiser-Friedrich-Bad complex to the Heidenmauer. Foundations of a larger building at this location are still described in the 17th century; the surrounding meadows and vineyards bore the field name "Im Saal". In a plan from 1809/12, the hydraulic engineer Weber documents a rectangular floor plan in Saalgasse, half of which is built over, with the designation "Remains of the Kaisersaal". Another interpretation places the court in the castle district.

The royal court was the administrative and economic center of the royal estate in and around Wiesbaden. The large imperial forest of the Taunus forests, the "Höhe", belonged to this district, as did the parish church, which in its beginnings must be considered a royal church.