Demon of the Well
E3M9 -- Warrens - 100% Kills / 100% Secrets
The quintessential secret map from the original game, this is probably the map that solidified the idea that a secret map should be strange, gimmicky, or heavily conceptual in some way. 'Fortress of Mystery' is of course also a heavily conceptual map, but in all honesty is mostly just a little novelty fragment, and its overarching concept can be comfortably ignored entirely on a continuous playthrough (which seemed to be the norm in the game's early life). I don't personally find it objectionable when a WAD's secret maps are essentially just 'regular' maps (or indeed, when an author makes them part of mandatory progression in order to give himself another mapslot or two to work with), but I do usually look for them in hopes that they'll be a bit oddball in some way, and that began with Warrens. It's perhaps difficult for a more recent audience to conceive of the map's potential impact, since understanding of how secret maps work/where they're positioned is generally common knowledge now, but back in the day this was a pretty cheeky move on id's part.
I've fought so far into Hell and overcome so much, so why am I back in Hell Keep again? Is my game glitched? Is this some kind of cruel joke? Do I really have to fight all the way back through Hell again? At least I still have my weapons...wait...
The first time you see it it's easy to miss the little differences that indicate that it's not, in fact, exactly the same map as E3M1 (incidentally, I wonder if it might've been better if it were an exact one-to-one copy up to the halfway point...I can see arguments both for and against this). One's liable to wander through what is by all appearances Hell Keep all over again with a steadily mounting sense of dread (not necessarily of imminent player death, but rather that something has gone horribly awry with the episode in general), and then the big room after what was the exit teleporter is quite a shock. That is, a shock simply by virtue of existing in the first place, and even more of a shock because OHFUCK CYBERDEMON!! It lives! Never mind that the cyberdemon's pretty easy to beat here, it's a moment I'll always remember. Same with the rest of the stuff behind the facade in Warrens, really--pretty hammy combat (though the surprise teleport into the combtooth area with the cacos and many shotgunners is kind of cool), nothing very deep, but it's that initial shock that matters.
E3M7 -- Gate to Limbo - 100% Kills / 100% Secrets
Ah, Limbo, bit of a soft touch. A fairly quiet, sedate map as far as combat goes, this is another of those maps which favors atmosphere over fast action, although in this case I suspect that's more because the concept underlying the map didn't leave a lot of room for bloody setpieces. This is just fine, I think--Limbo (and its alleged Gate) seems like a place that would be rather dreary and languid rather than full of sound and fury, after all. Like Unholy Cathedral before it, the real obstacle here isn't demons but rather the puzzle presented by the map itself. Some seem to feel that having two maps centered on teleporter puzzles rather than action so close together in the back half of the episode makes Inferno feel imbalanced and too much of a departure from the rest of the game, but I've never shared that view--the more 'supernatural' bent of the progression in the E3 maps fits the overarching theme quite well, I think, and Limbo has never felt terribly similar to Unholy Cathedral in terms of how it plays, to me. Incidentally, I think the first time I ever played Limbo it did take me a while to work out how to reach the exit, but it's never given me problems since then. It's really not terribly obtuse, and there are so damned many radsuits down in that red maze that most players should be able to brute-force through it, just trying teleporters blindly until they hit the exit, if all else fails.
I don't want to totally downplay the combat in this map, mind. The actual monster count is rather low, of course, and on a continuous run they hardly register as a factor at all. Quite a different story from pistol-start, though--the surprisingly nasty shotgun greeting can drain your health pretty well when all you have to work with is a pistol, and the ammo balance here is very tight by IWAD standards--there are several cells and the like, sure, but you don't get a plasma rifle until the map's mostly done. You do most of your fighting with the shotgun (there is no chaingun, and little bullet ammo), and shells are rather limited, especially considering that the rocket launcher is heavily guarded and has very little ammo available. Infighting to the rescue, of course; no berserk pack here. Still not very hard (even 'back in the day', I suspect, though I didn't pistol-start this map until years later), but this sort of vaguely underarmed feel to the action also seems to suit the 'Limbo' theme, somehow.
E3M8 -- Dis - 100% Kills / 100% Secrets
DooM's much-vaunted anti-climax. The poor spider mastermind isn't much chop on continuous play.....he probably freaks new players out just enough that they A) begin emitting various high-pitched screeches of general disapproval at the sight of him and B) begin discharging BFG rounds at him in a steady stream, which of course kills him in short order even from a distance, given his far-from-indomitable HP. I don't have a negative impression of my first time finishing the game--I distinctly remember being mildly traumatized (<--I mean that in the most heartwarming way possible) by the decidedly downbeat ending--but the actual final battle itself didn't really register with me, other than that the spider had a scary face.
This map really only works well from pistol-start. This is largely because the BFG is not an option in this case, of course, but more generally because the thing placement allows the mastermind a much better chance of winning the battle of attrition against the player; because there are absolutely no health aids, the spider's gatling gun is considerably more formidable, especially since the limited weaponry available precludes the possibility of an early knockout. All that being said, it's still not much harder than the other boss maps. The three other monsters present on UV seem purpose-placed to distract Daddy Longlegs while the player gathers up rockets and gets the plasma rifle and armor from the central enclosure; once you're armed, the battle is essentially won, unless you're hapless enough not to understand the concepts of cover and line-of-sight. So, yeah, it's still kind of anti-climactic even this way, but at least the battle's a little more relevant. Shame the poor mastermind's become a little bit of a laughingstock as far as PWADs go, but he's just not at all versatile, and needs for everything in his environment to be just so in order for him to be anything more than a big meaty nuisance. Some have said he needs a stronger weapon and more HP; for my part, I think what he needed was simply more movement speed and a less comically gargantuan blockmap. But alas, 'twas not to be.