Saarbrücken's schlossfelsen is a natural new red sandstone plateau. 240 million years ago, the sandstone was formed from sediments which solidified under high pressure. The stone is weather-resistant, but it is characterised here by being easy to work. The plateau slopes gently eastward, but in the north it has a steep cliff dropping down to the Saar – an ideal location for fortifications and other constructions over many centuries.

New red sandstone, mined in many quarries, was much in demand as a building material. On the schlossfelsen too, artisans – quarrymen – broke the stone out of the bedrock to build the castle moat and, later, to widen and deepen it. Having said that, the visible outer wall of the bastion consists of carefully carved stones which were delivered from elsewhere.

You are now in the middle of the moat, which was formerly part of the fortification of Saarbrücken Castle. At one time, it was out in the open. The castle and the later renaissance residence sat enthroned some 14 metres above this moat.

In the 18th century the moat was filled in when the baroque residence was built. The area beneath the Schlossplatz, accessible today, was excavated between 2003 and 2007 and covered with a concrete ceiling. Since 2007, this exhibition area has been part of the Historisches Museum Saar.