"Tarzan, Lord of the Jungle" (Tarzan #11) by Edgar Rice Burroughs

Cruel slave traders had invaded the jungle of Tarzan of the Apes. Now they were headed toward a fabled empire of riches which no outsider had ever seen, intent on looting. And toward the same legendary land was stumbling the lost James Blake, an American whom Tarzan had vowed to rescue.
Following their spoors, the ape-man came upon the lost Valley of the Sepulcher, where Knights Templar still fought to resume their Holy Crusade to free Jerusalem.
Soon Tarzan, true Lord of their ancient motherland, was armed with lance and shield, mixed into their jousting and ancient combat.
It was then that the slavers struck!