The ToC lists 10 levels (and several sub-levels in the form of 1A, 7A, 8 East etc.), 8 appendices, with the 8th being the map pack. The Introduction section is pages 3-9, with "a brief history" taking up 3-4. The Brief Overview of Gunderholfen reads: "Gunderholfen consists of three main sections: levels 1-4, which are not connected to the Central Shaft; levels 5-10 which are connected to and lead off from the Central Shaft; and the Floating Level which is a separate demi-plane accessed through the Duergar settlement on Level 7."
History: The long and short of the history is: Gunderholfen was a dwarven city. One day they all disappeared. For one thousand years, evil creatures fought over the stone halls, while men forgot about them. Eventually a human wizard consolidated power in a large portion of Gunderholfen. He found a demon and imprisoned it. The wizard died (?) in the process.
The rest of the intro section covers general knowledge (non-locals, locals, and educated), a 15-entry rumour table (rolling 16-20 indicates no rumor), an incentives and patrons entry. This brings me to the most useful piece of the whole book: Tasks, Rewards, and Possible Patrons chart. The chart lists 14 possible tasks that can be easily imported to any megadungeon (just change macguffin names). Next, Hawkins gives advice on integrating plots and NPCs. Largely, this is useless, because every entry directs you to a different part of the book. A short section for notes for the GM follows next, with a welcome abbreviation list.
Hawkins lists their Entry Layout (which seems unnecessary), followed by a design philosophy before diving into level 1. In the introduction, the town of Longfelt is discussed, but there's no real write up until Appendix 4. Longfelt should be close to the front of the book, and set up as the home base. Having it as an appendix is a weird choice.
NOTE TO MEGADUNGEON DESIGNERS: Put a map with your chapters for your levels. Especially in the same book. There are exceptions to this (Barrowmaze), but for the most part, the map and key should be together. Print the map twice if you need to (like Castle of the Mad Archmage) but keep them together.
Also put your wandering monster tables with the level they correspond to. Don't make an appendix at the back of the book. I don't want to flip through 300 pages to get to a chart. Using the PDF is even worse. A 400 page PDF should have bookmarks, especially when the information is disjointed.
Level 1 (A-C): Each level begins with a common description (in this case: "Unless otherwise noted, the walls are made of roughly hewn sand-colored stone. The ceilings are 12’ high in passages and 18’ high in rooms. Doors are of heavy wood with iron pull rings set on the right side of the door. They open outwards from rooms.") (Gunderholfen pg 11).
I am not sure why the author decided to break level 1 into three parts. I am guessing it is primarily to facilitate the maps being standardized. Level 1A is the domain of kobolds, and they pervade the majority of it, with rats, a ghoul and a fungus forest being outside their influence. Level 1B is full of goblins, including the entry where they have a nasty trap set up. I get the feeling that the goblins and kobolds fight for territory, but it is not stated. Level 1C is separated by a portcullis and leads to a hobgoblin area.
The routes to the next area for the kobolds and goblins are quick (>5 encounters) but if characters make a wrong turn they are stuck in a loop that dead ends or circles itself. (However, there is an alternate exit of the dungeon in the goblins area.) The hobgoblin area is similar (5 encounters to the stairs). The route through the hobgoblin area requires going through their jail. Another dungeon exit is near the stairs (whew!). These maps don't facilitate exploration or verisimilitude. Overall, the entire level lacks imagination and feels like 3 smaller dungeons lashed together.
Level 2: Largely, level 2 is your standard low level cave adventure, with stirges, bugs, giant fauna, flora and a few unique NPCs thrown in. Overall, the map is slightly better than level 1 (at least it is a single level).
Level 3: Finally some choices in exploration! There are 3 main veins through the level, and they had the ability to be more interconnected but were not. (If running this book, I would fix that.) Level 3 starts in a room with 3 exits, and has several monsters within. Notably, the kiigoths (new monster) and ogres are engaged in long term warfare within the middle of the level. Largely there aren't a lot interesting dungeon defining areas (puzzles, traps, lore or other unique features). The Sun Face (a puzzle that gives rubies and fire gouts) is the sole exception, in my opinion.
Level 4: (Why are there 100' between levels? Seems weird.) Level 4 is broken into 4 parts: A-D. It is massive, including a defunct dwarven town and an active "monster" village. Ogres, "impoverished orcs," some demonkin, and a few of the common dungeon denizens (bats, lizards, etc.) populate 4A. Again, all of the rooms are basically encounters or empty.
4B is the defunct dwarf town, now full of undead. Now, I like the concept. And it is passible. But, it is all standard fare. There's no artifact powering the undead, there are no boosts. It's all what-you-see-is-what-you-get. The one exception is the Hall of Animation. It's not often that a wight just watches the party and refuses to move. . .
4C is the "monster" village. Dark stalkers, creepers and their like inhabit nearly the entire sub-level.
4D is an area that archmage used frequently. Out of all of the levels in Gunderholfen, this is my favorite. It still isn't laid out well (map-wise), but there are several interesting rooms and secrets to find. A library, a hall of many (magical) thrones, a room of crystals and others finally make the players think through some rooms (opposed to brute force). There are plenty of combats too including gnolls, owl bear (monster condo!), otyugh (5 pages after their artwork), and a grell (called a grold).
Level 5 introduces the Central Shaft (finally a quick-access to multiple levels). Level 5 is 200' below level 4. (for those of you keeping track, it is now 560' below ground.) These levels really begin to blend together. Very little makes them stand apart from one another. Minotaurs and a different wizard populate areas here, with hill giants nearby (again, 500' below ground.) There is a crypt section that houses a bunch of undead and some treasure.
5 East: Overall, the level is more of the same with doppelgangers being the "faction" of the level. A few more undead, vermin and little else that's exciting. Although there is a 1-way teleporter to get the group into deeper trouble, so that's nice.
Level 6 has two ingress points (thanks to the Central Shaft). We are seeing a bit more agency for the players' exploration. The spawn of Sethid is a contingent (on time) encounter that might be interesting. They appear only at night (even 660' underground). In order to stop them from re-appearing night after night, all of them must be slain on the same night. It's a fun mechanic that is wasted so deep in a dungeon.
Overall, Level 6 does a much better job of making rooms interesting (a candle that never goes out and prevents turn undead and a sacrificial gong are two examples).
6A: Back to everything ready to kill the party without negotiation or nuance. A few humans (leveled party) and a derro band reside here as well as zombies. It again feels like everything is just waiting for the adventurers to show up.
6B: Troglodytes! Deep ogres! Qeel men (eel peeps)! At least the map is cavernous to break up the monotony.
6C: Lizardmen, troll island, more qeel men and argoyles (aquatic gargoyles) live here. Also a kraken.
All of level 6 feels disjointed. It could have been an interesting level, with qeel men populating all 3 areas of the level. Instead, there are a lot of small factions that just feel ... haphazard. The qeel men should be a dominating force, with a walled tower and the ability to move through water and on land easily. Instead, they feel like the largest group of terra cotta soldiers.
Level 7 this time the book does not list the descent distance, but I will assume another 100'. Thrones on the exit to level 6 (at the Central Shaft) teleport to thrones here (and vice versa), so that's cool. Vines also connect the two areas (and level 8), so there's a non-magic way to move.
It is noted in the introduction to the level that the PCs could eliminate the leadership of the duergar on the level without raising an alarm (if a secret door is found and used to access a demi-plane). It's an interesting dilemma, but unless the party gets lucky (by finding a teleporter on 7A) they'll likely miss it. Instead they will probably engage in guerrilla tactics with the duergar. There are probably close to 200 duergar (I stopped counting at 130). The duergar have an arena that might be an alternate angle to deal with them.
7A is a level filled with ghouls, and tunnels throughout, but it is self-contained. Besides the ghouls there is a death mist and several altars. The only reason to adventure here is the loot that the death mist holds.
The Floating Level is an "air-filled, egg shaped demi-plane that contains several land masses." There are two ways out of the floating level: a magic item held by the duergar and a magic item held by a dragon within the Floating Level. Glak is a city established by the ubiquitous wizard at the top of the level, and is populated by humans and demi-humans. At least the inhabitants here have motivations and appearances listed below their stat blocks.
The remaining isles (in order):
B. The Demon Isle: an entire island of random encounters with demons.
C. The Faerie Isle: malign faeries and evil halflings (with a weird gender-split society) reside here.
D. Isle of the Cyclops: a cyclops and many "cyclopians" (basically cycloptic hill giants) live here.
E. The Hunting Grounds: another entire island of random encounters, this time with standard fare megafauna (buffalo, rhino, etc.).
F. Isle of the Dragon: is pretty self-explanatory. Jithax (the black dragon) has minions, and at one point attacks with a fly by breath weapon. From there, Jithax returns to her layer and prepares for the battle. She has a flame of scrying that allow her to watch the party's approach. She uses enlarge on herself and waits. This is a dangerous encounter and should be understood by the DM before running it, as Jithax has a lot of possible strategies and actions (and minions that join the fight). And the PC's almost certainly must face her (to get the McGuffin to get out of the Floating Level).
It also feels weird that there is a gate back to the first island within her lair. Why isn't she terrorizing the inhabitants of Glak on a daily basis for riches?
Level 8: This level is primarily controlled by the Suleb Darn, an organization of "powerful anti-heroes." There are a few neat tricks here: moving a statue part to disarm a magical fear effect, pushing another statue part to open a secret door (it's always nice to know the secret door triggers). There's a great illusionary trap too.
The humanoid anti-heroes are back to having no listed motivations, but they have mostly well thought out tactics. It is weird that the fighters are together, the magic-users are together, etc. It seems like these could become very boring fights.
Level 8 East: Queue obligatory cult level. Snake cultists are so 1982, and they don't even have James Earl Jones. They even have a medusa ("gorgon"), naga, and not-yuan-ti (serpent men). And there is a small werewolf area. Because those two groups won't wipe each other out.
Level 9: The wizard's library is held here, as well as "the Beings of Nahr." The later has a monster entry that is about 3 pages long and can use up to 7th level spells. The guardian of the library is a foo dog, and a neat NPC.
The true threat of the level is the Beings of Nahr. They control a significant portion of the level outside of the library.
Level 10 A: is a not present demi-liches' lair (a deathgazer, the creature on the cover, has moved in). The deathgazer actually has text detailing how it would parlay with the party, and it has an interesting combat (an artifact that can be attacked to weaken the deathgazer).
Level 10B: is the final revelation of what happened to the ubiquitous wizard. I will just leave this level at that.
So, my honest take: this is the worst megadungeon on the market. It is largely because G. Hawkins assumes where the party will fight, and where the party will negotiate. Further, there are a lot of group politics that just don't feel fleshed out to the point of believability.
Most levels feel like rehashes from other sources (snake cult level, demi-liches lair, goblin warrens etc.). This is fine if it feels sprinkled in. But, in Gunderholfen's case it feels like that's the majority of the levels.
There aren't enough unique encounters for the size of the book. A few levels stand out as interesting, but for the most part, it is all stuff that I can find in Undermountain, CotMA or other kitchen sink dungeons that do it better.
Layout is bad. Put a map somewhere accessible to the text it relates to (even a small one like CotMA). Why are the wandering monster tables an appendix in the back? A lot of illustrations were not near their rooms. It just is sloppy. There are 3 player handouts. They are all on the same page.
The flow of the maps is poor. They need some additional interconnectivity. Player choices feel stale currently. Additional points of ingress to the dungeon would be helpful.
All that said, there are some strong points to the book too. I love the artwork. It truly evokes the early days of the hobby and I would absolutely love to make a book with that aesthetic. There are a few unique rooms that should be highlighted (and I did).
I commend G. Hawkins for putting this into the world, even if I think it misses the mark. It is difficult to put something out there, and 400 + pages even more so. 3/10
Even with that low rating, I love megadungeons, and do not regret buying this one (although I don't think I ever will regret buying a megadungeon). I will never play it in its full form, but I will poach for my home game and enjoy it for what it is. If you get it, buy it on sale, or grab the PDF and skip the dead trees.