In the 1960s, the construction of the urban motorway and a relief road with two lanes in each direction – designed to carry the traffic in times of flooding – led to massive interventions in the area all round the Schlossberg. Among other things, the plateau along the Schlossmauer was to be moved back by no less than 17 metres! That resulted in the partial dismantling of part of the castle gardens and the bastion up above the Alte Brücke, and above all the loss of the retaining wall designed by baroque master builder Friedrich Joachim Stengel. There was fierce public debate, but the building work began in 1962.

From the very beginning, the earthworks were under the supervision of the State Office for the Preservation of Historical Monuments; after all, this was a matter of documenting irretrievable material for research. The work revealed a gallery, cut into the rock, with a vaulted sandstone ashlar ceiling and a steep downhill gradient. Just a short distance away, some foundations of the garden wall were found, as were the two inner stair towers of the so-called summerhouse dating from 1577, digging for which had already been begun once before, in 1938.