The Dragon of the Ishtar Gate

This is an adventure novel, set during the reign of Xerxes, Originally published in 1961, it was the third written, and earliest in chronological order, of five loosely-related historical novels. The others are: "An Elephant for Aristotle" (1958), in the reign of Alexander the Great, involving a trek across Asia with a rather large gift for the philosopher; "The Bronze God of Rhodes" (1960), in which the background is the struggle among Alexander's successors, with the construction of the Colossus of Rhodes only one of the incidents; "The Arrows of Hercules" (1965), a story of war and military inventions in the time of Plato; and, finally, "The Golden Wind" (1969), concerning sea-borne trade and exploration in Hellenistic times. 
"Dragon" is probably closest in mood to de Camp's fantasy novels, while "Bronze God" and "Arrows" are closer to his science fiction, with their description of engineering problems, and personality clashes among researchers and engineers. "Dragon" also reflects de Camp's then-recent acquaintance with the Conan stories and other works of Robert E. Howard. Bessas, the huge Persian nobleman, is not only a close match for Conan in physique, but his mood swings, fixation on his mother, and tendency to toss of verses, seems a reflection of Howard himself. (Fortunately, his poems reflect de Camp's verbal dexterity and wit, not Howard's melancholy.)