What a lot of gamers forget, assuming they ever actually played D3, is that there is absolutely no plot to the module, just as there was no plot to its precursors in the D series. The "plot" of the series, such as it is, mostly occurred in modules G1, G2, and G3, where the evil high priestess of the Elder Elemental God, Eclavdra, was attempting to organize the giants into a vast army with which to subjugate a portion of the surface world, in the process gaining power for herself and her house, Eilservs. Once that plan is defeated, though, all that remains for the PCs is vengeance and exploration of the depths of the earth. Eclavdra -- or her clone -- reappears in Vault of the Drow, but only as the leader of House Eilservs, not as "the big bad evil guy" of the module. No such personage exists in D3, as its 28 pages are devoted primarily to describing Erelhei-Cinlu, its inhabitants, and their activities.
There's a lot to love in this module, though I admit that its hard to erase from my memory the horrible ways in which the drow have been fetishized and bastardized in the years since. It's frankly a testament to Gygax's brilliant imagination that he made chaotic evil elves who (mostly) had a thing for spiders so alluring. And of course, in 1978, when this module was first published, the drow were new and exciting rather than clichéd and dull. I know I found the drow fascinating back in the day, even if I never quite shared the same level of interest that many did (the same goes for elves generally, so maybe I'm weird).
Erelhei-Cinlu itself is like a pulp fantasy come to life, illuminated by the soft purple glow of phosphorescent fungi and filled with buildings built on the presumption its inhabitants could naturally levitate, it's an alien place, where the PCs can't help but feel like fish out of water. Even more unsettling in my experience is that, unless the PCs are actively disturbing the peace of the city (or have ticked off someone of importance), they can wander about the place without being hunted down like dogs. True, it's a chaotic evil city and it's all too easy to wind up on the wrong side of inter-house disputes, but the drow are civilized and their city behaves according to rules, albeit twisted and evil ones.
The tiers and dungeons of Erelhei-Cinlu reek of debauchery and decadence, and the city‘s inhabitants are degenerate and effete. (Those with any promise and ability are brought out of the place to serve the fighting societies, merchant clans or noble houses. The rest are left to wallow in the sinkhole of absolute depravity which is Erelhei-Cinlu.) The most popular places in the city are the gambling dens, bordellos, taverns, drug saloons, and even less savory shops along the two main streets. The back streets and alleyways too boast of brothels, poison shops, bars, and torture parlors. Unspeakable things transpire where the evil and jaded creatures seek pleasure, pain, excitement, or arcane knowledge, and sometimes these seekers find they are victims. All visitors are warned that they enter the back streets of the city at their peril.It's easy to see why the drow made such a profound impression on gamers. What Gygax has done here is present us with an entire evil city to use as our sandbox, pursuing whatever adventures we wished within or without its walls. It's a great example of location-based design and a reminder of what modules were like before the demands of convention play or obsession with "story" changed their nature forever.
When I get around to it, after all my various other projects are put to bed, I'd love to take a whack at designing something like Erelhei-Cinlu. If it's even a tenth as evocative and useful as what Gygax achieved, I'll be beside myself with joy.