Being the first region in the dungeon this is the biggest hurdles for groups trying to tackle the WLD. It's going to determine if any of the players are willing to take on the rest of this behemoth. Unfortunately it's also full of flaws and this region is probably the biggest reason why most attempts at tackling the WLD seem to peter off before they get anywhere.
Before tackling Region A I'll take a brief look at Map Zero.
So, here we have the corpse of a dead titan who once guarded the entrance to the dungeon. It's hardly essentially but personally I would suggest removing it. First and foremost it's a distraction. You've got an unexplained giant corpse, a gigantic magical halberd and suit of armor and 500 pounds of notes and doodles. If my players stumbled across this they'd probably waste half a session trying to unravel the mystery and purpose of this odd corpse.
The biggest problem with this encounter is that there is simply no point to this encounter. The players have no chance to learn why the titan is here (the notes are not detailed, apparently it never occurred to the writers that the players might have access to the Comprehend Language spell) or what happened to him (the Word of God is that he died of old age, which seems ridiculous for a titan even more so since he apparently took no steps to get a replacement). It provides no hint to what waits for them in the dungeon, no clue as to the plot. When taking on the WLD it's important to build up momentum to plow your way through and this certainly isn't going to help.
This is just a mild niggle but since I'm here I'll go ahead and say that it'll probably be better to avoid the "ominously-pitch-black-door" approach in A1. Simply have the players pass across the barrier around the dungeon at some point in the tunnels. It doesn't matter when but if the players attempt to turn around and leave the dungeon they find that there's an invisible barrier blocking them.
Again, the main reason is to build momentum and avoid speed bumps. If your players find a door that opens into impenetrable black nothingness what do you really expect them to do? On top of that it just doesn't make any sense in the context of the dungeon. A1 was never the original entrance to the dungeon. It was just a random room in the far corner of the place. So why would the dungeon's magic barrier cover every other room but not this one?
Before we go room-by-room lets take a general overview of the region. Let's start off positive with some compliments.
Although there are a lot of significant problems with this Region it does have a lot to recommend it. The Region's plot is moderately interesting and the backstory is fairly well thought out. Even better, the Region is more than just a series of room-to-room battles with low-level enemies. There are many enemies who are too frightened to fight or are willing to talk. The warring humaniod factions provide plenty of room for role-playing, moral choices and alliance building. Considering that the fairly restrictive nature of the WLD this region has almost no rails and is big enough that you could even consider it kind of a sandbox adventure.
So, with the praise out of the way let's get to the problems. We'll start with general issues.
Region A is, for the most part, is terribly dangerous for the low-level players it's meant to be used for. This will be a major part of the corrections I'm going for. I'll mark these adjustments to make it clear that I'm correcting for balance purposes. The general assumption is that CR and balance should assume that the party consists of 3-5 players with all of the major roles (healer, spellcaster, warrior, rogue) filled to some degree or other. Obviously your party might be very different (some people tackle the WLD with much larger groups, gestalt characters, lots of henchmen, etc). In these cases you might want to ignore my Balance corrections or even make encounters harder.
Traps are a particularly big problem here. Most of the traps are magical which means that it's difficult to impossible for rogue's to find (let alone disable) these traps. Although it's practically required that you bring a rogue into the WLD they're going to feel very useless in Region A since they have basically no chance of helping out without several levels under their belt.
Some GMs love racking up player kills and some players love to charge head first into the meat grinder. After all, if there wasn't appeal in these games then the Tomb of Horrors wouldn't exist. But for most people the WLD should not be a meat-grinder dungeon. The goal is to get through this mammoth complex not die on the doorstep. Even worse the design of the WLD makes it extremely difficult to justify getting new players to join up with the party. Isn't it more satisfying to run a single character from level 1-20 instead of going through two or three replacements every Region? I'm not saying there shouldn't be risk or challenge but death shouldn't be a commonplace occurrence for players.
If you look through reviews of the WLD you'll probably find the biggest complaint with Region A is variety. Although the humanoid enemies are fairly varied and interesting the majority of enemies in this region are fiendish creatures that have poured out of a portal deep in the dungeon. That might seem fairly interesting but about half of these creatures are fiendish darkmantles and the other two thirds are made up of fiendish rats of varying sizes and fiendish stirges. That's about it. Now, the most common defense is that there are only so many low-CR monsters to go around. But that's simply not true. Here's the low-CR animals and vermin (which are not used anywhere else in the dungeon) that I use to add some variety: Fiendish Boars (CR 2), Fiendish Hyenas or Wolves (CR 1), Fiendish Riding Dogs (CR 1/2), Fiendish Eagles (stats also work for other creepy birds, like giant ravens, CR 1/2) and Fiendish Constrictor Snake (CR 2). There's more beyond that but these fit the theme best.
It doesn't help that they choose perhaps the most annoying creatures to fight in large numbers: Darkmantles (whose darkness and grapple ability makes them terribly irritating), stirges (capable of inflicting Con damage) and rat swarms (resistant to weapon damage and their fiendish nature makes it tough to hurt with magic).
Theme of the Region
Although the players are unlikely to know anything about the true nature of the WLD at this point they may start to ask questions later on. In addition, the inconsistencies with Region A bother me personally as a GM so I'll go ahead and address them. Supposedly this region is part of the dungeon designed to imprison ancient supernatural evils and built by angelic immortals. So why are there random caches of long-rotted supplies scattered about? Why are there magic traps scattered at random which would be completely ineffective at dealing with any demonic prisoners?
My personal explanation? Since the Celestials were unable to contain the demons and devils spawning in the dungeon and the undead drawn by the rotting corpse of a slain god they called upon mortals to help by expanding the dungeon. Region A was created by mortal hands (although with celestial inspiration) and the traps and prisons here were created with the mortal's relatively flimsy resources (thus why none of them have succeeded at containing their prisoners). The remains of food or barracks scattered around were abandoned long ago (the mortals have died of old age or risen as undead in some deeper region of the dungeon.
I'll only mention places that I'm changing. If a room doesn't appear then it remains unchanged.
As I noted earlier I suggest you ignore the "door into darkness" and have the PCs already pass through the WLD's barrier well before they arrive in this chamber.
The very first trap players are likely to run into is a magical Burning Hands trap which guards a mass of black mold (basically another trap) and then some more black mold, then a magical acid arrow trap (something quite capable of taking down an unlucky 1st level character). All of this guards...some old stoneworking tools and rotten rope. Personally I don't see much point so I say have the traps already triggered and exhausted (either long ago or by the orcs). If you'd like to start out with a trap then replace the Acid Arrow (which is DC 27 to disarm for god's sake) with a basic arrow trap that's only good for 1d4 more attacks before being exhausted.
The orc here was apparently killed by stirges but my recommendation is to switch it to rat-bites. The rat swarm is not only closer but it's a much bigger threat to low-level PCs. A bit of warning would help them out immensely.
Personally, Region A has made me hate darkmantles so for this I'm getting rid of almost all of them. They're terribly annoying opponents to spring on low-level PC, even more so when combined with the fiendish template's Spell Resistance. Instead I recommend a pair of large, evil-looking ravens (stats as a Fiendish Eagle) feeding on an orc corpse. Rather than fleeing as the PCs approach they'll attack. This odd behavior should clue the PCs in that something's up.
Now if a Fiendish Darkmantle is annoying for a group of 1st level characters a Fiendish Rat Swarm is just plain unfair. The thing takes half damage from most weapons, has more hit points than the party tank and can attack multiple characters at the same time, automatically hitting. On top of that apparently the writers of the WLD got a little bit creative when applying the Fiendish Template (that or they screwed up which seems equally likely considering they got the swarm's HD wrong in the Smite Good and Spell Resistance portion of the creature description) and they thankfully ignored the fact that it should have Damage Reduction but did replace it's Filth Fever with Devil Chills (which is terrible for low level PCs, requiring 3 successful saves in a row to get rid of). The best way to deal with a swarm is torches or alchemist's fire (is ineffective due to fire resistance) and magic. Unfortunately due to swarm traits and energy resistance the fiendish swarm is literally immune to every 1st level attack spell in the SRD. Even if your spellcaster does have something else then the swarm's SR of 9 means there's a 40% chance it won't do anything.
So here's my suggestion. First, give the PCs more than one round to react to the swarm. The thing doesn't move very fast after all. Let them get a few chances to hear the swarm and hopefully prepare. Next, make the swarm a mundane rat swarm that has been gathered by a single, fiendish rat. The rat is hidden in the center of the swarm which grants it cover. If one of the PCs spots this unusual rat with burning red eyes (Hide Bonus is +10) and kills it then the swarm disperses. This drops the CR of the encounter from 3 to 2.
Getting late and this takes quite a while so I'll go ahead and end here and resume Region A in a future post.