The Hamamni Baths were built in the 1870s by Sultan Barghash on the model of a Persian hammam, with hot and cold rooms, steam chambers, an under-floor furnace, and barrel-vaulted ceilings pierced with star-shaped openings to let in dappled light. They were the first public baths in Stone Town and ran until 1920.
What remains today is a small unfurnished ruin that you can wander on a cheap entry ticket. The custodian, who has the only key, will switch on the lights and walk you through the rooms with a short history; the place is best appreciated by understanding what each space was used for. Ten minutes inside is plenty, but it slots well into a walking route from the slave market to Darajani.