Crusades

The Crusades were a series of conflicts fought in and around Palestine in the 11th, 12th and 13th centuries. The principal aim of the crusades was for European Christian forces to retake control of Jerusalem and the Holy Land from Muslim occupation, as well as campaigns against other races and religions. Monarchs, nobles and commoners, all involved took oaths to uphold the principles of the crusade in exchange for indulgences against past sins. (DW: The Crusade, The King's Demons)

The Doctor, Ian, Barbara and Vicki visited Palestine during the Third Crusade and met several important people involved in the conflict. (DW: The Crusade)

Later, when the Doctor's fifth incarnation arrived in England in 1215, he was able to determine that an imposter had taken the place of King John, since the real king was known to be in London taking the crusader's oath. (DW: The King's Demons)