On the outside of the semicircular building, the course of a former water ditch from the 12th century can be recognized. This was established as part of the castle-like fortification of the imperial palace by the Staufer, was 16 meters wide and over five meters deep. As an element of the military fortification, access to the semicircular building was also bricked up via the imposing "Heidesheimer Tor." A paved path today retraces the early medieval route to the originally tower-flanked gate. The green area with a playground and the road above it, "Auf dem Graben," are still oriented today in their semicircular form to the monumental semicircular building of the imperial palace. It had a diameter of
89 meters.
Opposite the Heidesheimer Tor, an herb garden was created in 2012. This is filled with useful plants and fruit trees that are already listed in the Capitulare de villis, the estate regulation of Charlemagne (747/48−814). The Capitulare served as a guide on how to manage a royal court to always be prepared for the visit of the constantly traveling ruler and his entourage.
