An entrance in the side of the Altes Rathaus flanking Schrottgasse takes you to the Grosser Rathaussaal, where large-scale paintings by 19th-century local artist Ferdinand Wagner show scenes from Passau's history with melodramatic flourish. You can also sneak into the adjacent Small Assembly Room for a peek at the ceiling fresco, which features allegories of the three rivers.
The rest of the Rathaus is a grand Gothic affair topped by a 19th-century painted tower. A carillon chimes several times daily (hours are listed on the wall, alongside historical flood-level markers).