The Citadel of Fear is a lost world story and focuses on a forgotten Aztec city, which is "rediscovered" during World War I.
Excerpt:
"The sun, he thought, had grown monstrous and swallowed all the sky. No blue was anywhere. Brass above, soft, white-hot iron beneath, and all tinged to redness by the film of blood over sand-tormented eyes. Beyond a radius of thirty yards his vision blurred and ceased, but into that radius something flapped down and came tilting awkwardly across the sand, long wings half-spread, yellow head lowered, bold with an avid and loathsome curiosity. "You!" whispered the man hoarsely, and shook one great, red fist at the thing. "You'll not get your dinner off me nor him while my one foot can follow the other!"