Duke William IV decided to expand Ingolstadt into a state fortress. The expansion took place from 1538 to around 1545 under the direction of Reinhard, Count of Solms.[1] The reasons for this were the strategic importance of Ingolstadt at the crossroads of the roads from Regensburg to Ulm and from Nuremberg/Amberg to Augsburg/Munich, its location on the northwest border of the duchy, and the need to control a bridge over the Danube. The nearest Danube bridges in Neuburg, Regensburg, or Passau did not belong to his domain. The basic idea was to build a brick earth wall with a moat in front of the medieval city wall, on which cannons could be placed for defense. Brick bastions were built at the corners to reinforce the city and allow the moat to be fired upon and the neighboring walls to be protected by flanking fire. The following structures were built during this period:
Brick Bastion, masonry and casemate bastion
Harder Bastion
Spherical Bastion
Kreuztor Bastion
Roundelle at the Women's House
Strafwehr (Smoke Hole)
Roundelle at the Red Tower
Danube Front
The masonry structures of the first three bastions are still preserved today, although the Spherical and Harder Bastions are still in use. The remaining structures have not survived, but are all documented in Jakob Sandtner's city model from 1572/1573.
Source: Wikipedia