[1] Although the island is small, its strategic position at the limit of navigation for ocean-going ships in Africa's largest natural harbour made it an ideal base for European slave traders.

[6][7][8]The early phase of the castle's history ended in 1728 when Bunce Island was raided by José Lopez da Moura, a Luso-African slave trader based in the area.

He was the richest man in present-day territory of Sierra Leone, the grandson of a Mane king and part of the hybrid Luso-African community that had developed along the lower rivers.

[9] Mixed-race men from such families as the Caulkers, Tuckers and Clevelands sold slaves and traded goods at Bunce Island.

[9] The slave ships came from London, Liverpool and Bristol; from Newport, Rhode Island in the North American colonies; and from France and Denmark.

The British traders rebuilt the castle after each attack, gradually altering its architecture during the roughly 140 years it was used as a slave trade entrepôt.

African farmers with rice-growing skills were kidnapped from inland areas and sold at the castle or at one of its many "outfactories" (trading posts) along the coast before being transported to North America.

Several thousand slaves from Bunce Island were taken to the ports of Charleston (South Carolina) and Savannah (Georgia) during the second half of the eighteenth century.

[14][19] American colonist Henry Laurens served as Bunce Island's business agent in Charleston, and was a wealthy planter and slave trader.

The chief negotiator on the British side was Richard Oswald, the principal owner of Bunce Island; he and Laurens had been friends for thirty years.

[14] In 1787 British philanthropists involved with the Committee for the Relief of the Black Poor in London established Granville Town, a settlement for freed slaves on the Sierra Leone Peninsula, 20 miles (32 kilometres) down-river from Bunce Island.

Bunce Island was shut down for slave-trading; British firms used the castle as a cotton plantation, a trading post and a sawmill.

Anthropologist Joseph Opala's research linked the island to the Gullah people of the United States Low Country.

[24] Historian David Hancock documented Bunce Island during the period of Grant, Oswald & Company in his study, Citizens of the World (1997).

[25] In 2006, television actor Isaiah Washington visited the island after learning through a DNA test he was descended from the indigenous Mende people of Sierra Leone.

[26] Washington later donated US$25,000 to a project to create a computer reconstruction of Bunce Island as it appeared in 1805, to mark the bicentennial of the abolition of the African slave trade by the UK and the United States.

[26] Project directors Joseph Opala and Gary Chatelain at James Madison University created a three-dimensional image of the castle using computer-aided design and historic drawings.

[7][28] Bunce Island is now protected by the Sierra Leonean Monuments and Relics Commission, a branch of the country's Ministry of Tourism and Culture.

Bunce Island has been called "the most important historic site in Africa for the United States"[4] because thousands of slaves were shipped from here to ports in the American South.