There’s no information about this structure at the site, but it’s known as Bastion Südallee, or the Bastion on the South Avenue. This street used to be the line of the city wall, but it was demolished in the nineteenth century and turned into a park.
The bastion, also known as the Red Tower, was saved from being demolished. It was originally built in 1543 with the tower being fortified and the lower level was used as a gun chamber.
Unfortunately, the downstairs section is only occasionally opened up to the public and it’s a shame that something a little more can’t be done with this structure.