Dalmadal Cannon is a heavy iron reminder that Bishnupur was a fortified capital, not only a temple town. The cannon is associated with the Maratha raids of the 18th century, when western Bengal was repeatedly attacked.
Local legend gives the gun a divine turn: Madan Mohan himself is said to have fired it to save the town. Whether you take that literally or not, the cannon adds a useful martial note to a day otherwise dominated by temple architecture.