For posterity, I wanted to note that there is another Conan story by R. E. Howard that also features a sleeping, drugged city ruled by an evil monstrosity (which Conan explores along with a woman): "The Devil in Iron."
He cradled the girl's glossy head in the bend of his heavy arm, and kissed her full red lips with unfeigned relish.
'You are strong,' she repeated, her voice waning. 'Love me—love—' The sleepy murmur faded away; the dusky eyes closed, the long lashes drooping over the sensuous cheeks; the supple body relaxed in Conan's arms.
He scowled down at her. She seemed to partake of the illusion that haunted this whole city, but the firm resilience of her limbs under his questing fingers convinced him that he had a living human girl in his arms, and not the shadow of a dream. No less disturbed, he hastily laid her on the furs upon the dais. Her sleep was too deep to be natural. He decided that she must be an addict of some drug, perhaps like the black lotus of Xuthal.
As the bolded passage points out, Conan notices, in universe, the parallels to "Xuthal of the Dusk"/"The Slitering Shadow."
(These two stories are possibly my two favorite Conan tales. This is not so much for their plots, which both rely on bizarre coincidences, but because they feature two of the most evocative eldritch monsters that Conan ever fights.)