This story is a blend of history and fantasy.
Trier remains a real city in Germany, home to the Porta Nigra, the cathedral, the basilica (known as the Aula Palatina), and surviving house forts—although we added a house fort in the Judengasse for Otto. The hypocaust under the basilica and the tunnels under the streets that Otto and Fritzi use to free the prisoners are also real, remnants of heating systems and aqueducts made by the Romans.
The Trier Witch Trials were also all too real. They began in 1581 and extended all the way to 1593, with an estimation of a thousand executions. One of the four largest witch trials in Germany and possibly the largest mass execution in Europe outside of war, the Trier Witch Trials were so impactful that news traveled throughout Europe, sparking a revival of witch trials as far as Copenhagen, Scotland, England, and eventually, the infamous Salem Trials in America.
While the hexenjägers, forest folk, and the Well are all fictitious creations, it is true that the Romans did not dare breach the Black Forest. The Forest was originally linked to the Celtic goddess Abnoba, and it marked a mysterious, dense border invaders didn’t cross, perhaps simply due to the tough geography…but we’re not ruling out a bit of magical protection.