…that you can see a large granite ball in the window of number 11 Höllgasse?

It is not a cannon ball but a catapult ball.
The Fortress Upper House was not just built for protection against foreign invaders. It was also needed as a retreat when the citizens of Passau rose up against their rulers the Prince Bishops. In the year 1298 it was particularly vicious. To control the mobs the Prince Bishop of the time ordered to bombard the town with granite balls like the one in the display. Yet that is just a small one. The largest one can be seen in the fortress with a diameter of 120 cm!!!