We might usefully begin to describe "The Forbidden City" as a throwback to an older kind of science-fiction adventure: the otherworld setting, or at least the core of it, is weirdly fascinating; the atmosphere, what few breaths we are given of it, is haunting and suggestive; the aliens are odd, presented with both humor and thoughtfulness; the wit is sharp but sparse; the adventure fast and led by a hero.
Now, the above describes many of the better science fiction stories of the field's earliest days, with strong representation down to the present. But it doesn't quite describe what we think of as a typical Retief story of the long-running series by Keith Laumer. All the usual Retiefian elements are in "The Forbidden City": wit and slapstick, sensuousness, fistfights, and so on. But what makes it stand out is what Laumer only sketches: a society of memorials and monuments; intimations of mortality, afterlife, and immortality; dwindled epigones of a race of heroes, attempting to commune with their great dead of eras long past. Although dirty dealings by the alien Groaci set up the local planetary situation, and Terran senior diplomats react with fawning incompetence, we have a thinner set of diplomatic bunglers of the Corps Diplomatique Terrestrienne (CDT), and correspondingly fewer counter-thrusts of Retief's wit.
The above would be plenty for a major novel, and we're given only a novelet's worth. The story leaves us with the impression that it's seen as through a screen darkly: we can make out the image well enough, but most of the pixels that could have been shining there are missing.
So my advice is to read Keith Laumer's "The Forbidden City" for the Jame Retief style of fast-thinking adventure amidst alien hijinks. And someday, go back to read it again for the seven-eighths of the iceberg which isn't written, but which we may think about.
The story opens:
An evening breeze bearing the fragrance of ten-thousand year old Heo trees in bloom moved across the Embassy dining terrace. In the distance pipes sounded softly, picking out a haunting melody, like fairy feet retracing a forgotten path through an enchanted forest. The setting sun, vast and smoky red, cast crimson shadows along the leaf-shaded streets below.