Sultan Battery is a compact black-stone watchtower at Boloor, built under Tipu Sultan in the late 18th century to watch the river approach to Mangalore. It is not a grand fort, but its position explains it: from the top you look over the Gurupura River, the fishing neighbourhoods, and the waterway leading towards the Arabian Sea.
The site works well as a short stop paired with the ferry or boat crossing towards Tannirbhavi Beach. Go for the river view and the military history, not for a museum experience; the structure is small, and the surrounding area is more everyday Mangalore than polished heritage precinct.