The Old Fort — known locally as the Old Slave Prison — is a square coral-stone fort facing the harbour, built in the 1860s when the Sultan of Zanzibar tightened his grip on the mainland coast. The thick walls held a garrison of Baluchi soldiers, and the underground rooms were used as holding cells for slaves waiting to be shipped across to the Zanzibar market.
The building is small but powerful — the stone-vaulted basement is intentionally disorienting, dark, and damp. The German colonial administration later used it as a prison; today it is an open monument with information panels. Allow thirty minutes. The fort sits a few hundred metres from the Customs House and is easily combined with that and the Boma on a single walk.