The Jahan Kosha Cannon is a huge early modern cannon near Katra Masjid, its name usually translated as "Destroyer of the World." It is one of Murshidabad's more compact historic stops, but it fits naturally into a route around the old Nawabi sites.
Come for the scale and the setting rather than a long visit. The cannon is best seen together with Katra Masjid, Fauti Masjid, and the palace area rather than as a destination on its own.