Boké sits along the Rio Nunez, one of the tidal rivers the French called the Rivières du Sud — the rivers south of Senegal that braided the Lower Guinea coast into a maze of waterways. Portuguese voyagers charted this stretch in the fifteenth century, but the navigation was treacherous, and for the next several hundred years European traders favored easier coasts to the north and east. That difficulty cut both ways. The very dangers that kept regular traders away made these hidden rivers a refuge for those who had reason to avoid scrutiny. The Rio Nunez became one of the last resorts of a trade that was, elsewhere, beginning to be driven out.

When slave traders were pushed from other parts of the Guinea coast, they retreated to these difficult, little-known rivers. In the Rio Pongo, at the small island of Matakong, and here on the Rio Nunez, they established themselves and built strongholds defended with cannon — the Fortin de Boké among them. Many barracoons, the brutal holding pens where captured people were imprisoned before being shipped across the Atlantic, were thrown up along these rivers in the closing years of the eighteenth century. The remoteness that traders prized was, for the people held in those barracoons, simply another wall — a place chosen precisely because the wider world was not watching.

For a long time France paid the region little attention. That changed after the Napoleonic Wars, when the British, working from their bases in Gambia and Sierra Leone, turned serious attention to the Rivières du Sud and the highlands of the Futa Jallon beyond. French interest sharpened too. The explorer René Caillié set out from this coast in 1827 on his famous journey to Timbuktu, quickening France's curiosity about the interior, and from 1838 onward French naval officers under Bouët-Willaumez mapped the coastline in detail. By 1849, France had stirred enough local resentment that it moved to establish a colonial protectorate over the Boké area. By 1895 the region had been folded into French West Africa.

The Fortin de Boké outlived the trade it was built to serve. Today it is the Boké Museum, and the transformation matters: a building raised to imprison people now exists to remember them, and to make sure visitors understand what happened on this river. The town around it has its own present-day life — Boké is the capital of its prefecture and a working river port, set in a tropical savanna climate of wet and dry seasons. But the fort remains the reason travelers come, a place to stand on the walls above the Rio Nunez and reckon honestly with the human cost the water once carried toward the sea.