The Tarzan Series

Tarzan and a young German find a lost remnant of the Roman empire hidden in the mountains of Africa. This novel is notable for the introduction of Nkima, who serves as Tarzan's monkey companion in it and a number of later Tarzan stories. It also reintroduces Muviro, first seen in Tarzan and the Golden Lion, as sub-chief of Tarzan's Waziri warriors.

With the speed of the great apes, Tarzan rushed through the jungle toward his home and family. But he was already too late. The marauders had been there before him. His farm was in shambles and no one was left alive. Of his beloved wife there was only a charred, blackened corpse, still wearing the rings he had given her. Silently, he buried the body and swore his terrible vengeance against those who had done this terrible deed. Then he set out grimly to track them—through warring armies—across a vast desert that no man had ever crossed—and to a strange valley where only madmen lived.

Tarzan finds an outpost of European knights and crusaders from a "forbidden valley" hidden in the mountains.

Tarzan had been betrayed. Drugged and helpless, he was delivered into the hands of the dreadful priests of Opar, last bastion of ancient Atlantis. La, High Priestess of the Flaming God, had saved him once again, driven by her hopeless love for the ape-man. But now she was betrayed and threatened by her people. To save her, Tarzan fled with her into the legendary Valley of Diamonds, while Jad-bal-ja, his faithful golden lion, followed. Ahead lay a land where savage gorillas ruled over servile men. And behind, Estaban Miranda—who looked exactly like Tarzan—plotted further treachery.