Imagine this: It is the year 1225, and a small merchant ship from Spain pulls up its sails and anchors in Algiers, in northern Africa. But the ship’s most important cargo is not olive oil or fruit. It is two Christian men who have offered themselves in exchange for other Christians who have been captured at sea by Muslim pirates. The men from the Spanish ship contact the Muslim rulers of the city, and the Muslims agree that they will give up the captives in exchange for the two men who have arrived.