On Dublin Island, the largest and most accessible of the three Banana Islands, the stone foundations and broken walls of former slave-trader compounds still stand amid the vegetation. The Portuguese established a significant presence here in the early 18th century, and the islands' sheltered coves made them an ideal — and terrible — staging point for the transatlantic trade. Walking among the ruins, some of which are half-swallowed by tropical growth, gives a visceral and sobering sense of what the coast looked like during the slave trade era.
An old British fort also survives on the island, its walls still partially intact. Together the remains form an informal open-air history lesson that few visitors to Sierra Leone even know exists. There is no formal entrance fee or guided tour infrastructure; the local community can often provide informal guides for a small negotiated fee. Take your time, and treat the site with the respect it demands.