Along the coast of Africa stand the grim remains of about 30 castles, or forts. These were the trading posts of the Trans-Atlantic slave trade. Captured Africans were held in the dungeons of these castles. Each dungeon had a door that opened to the sea through which the captive Africans exited to slave ships for their voyage to the New World ... and a life of slavery.

In 2010, Greg Merrell and Doug set out to document what remained of these historic, yet tragic structures. Over about a month’s time they traveled through Ghana, Senegal and The Gambia creating thousands of haunting images. Many of these were included in the exhibit -- The Doors of No Return -- at Philbrook Museum of Art.

A number of the photos of Africa's slave castles are on permanent display in the Smithsonian Institution's National Museum of African American History and Culture in Washington, D.C.