The biggest of the Mainz baroque forts was vastly destroyed during the siege of Mainz 1793, but rebuilt and extended until 1850. So it displays the development of underground fortifications from 1700 to 1900.
The underground vaults and galleries served as air raid shelter for more than 2000 citizens in WW2, witnessed by original markings, paintings and the memories of Mainz citizens.
Another unique feature is the thermal well of roman origin. Safety reasons restrict the visitors to 16 and the duration to 75 minutes.