On this edition of StudioTulsa, we speak with Doug Henderson, an architectural photographer based here in Tulsa who has photographed, starting back in 2010, many different forts and castles along the coast of West Africa where, from the 1600s to the early 1800s, European traders imprisoned slaves until ships could carry them to the New World. Through these grim and rather under-documented structures, more than 12 million people passed in their shameful journey to slavery. Henderson's images of these haunted, unsettling buildings -- there are about 30 of them still standing, and our guest today has photographed nearly all of them -- have been captured in recent years in a book ("Slavery's Castles") as well as a traveling museum exhibit ("Doors of No Return"). And now, several of these shots have been purchased by the Smithsonian's soon-to-open National Museum of African American History and Culture, which plans to keep the photos on permanent display in one of its slavery-themed exhibit halls.