You can’t miss the Postbahnhof, a long and slightly curved brickstone building, if you exit Ostbahnhof on the front side. In the former post train station conferences and occasional bigger concerts are being held.
But the majority of noise is being produced when you’ve almost passed the building on its right side. That’s the location of the Fritz Club, in two rooms that lay against the main building. The capacity isn’t much more than a couple of hundred visitors, big enough though for the many indie bands playing here. Bonus: towards the railway tracks is a nice beer garden in summer.