This is a very big (indeed, mega) dungeon crawl in a sandbox style with a bit of a twist. The caverns are spread out as an ancient sunken city in a canyon. There is a larger meta-plot for the players to foil. However, it may be quite difficult for them to figure out exactly what it is, given the scattered nature of the clues. In fact it can even be a bit daunting for the DM because of the organization of the book. A clearer explanation of the machinations of the villains and how the party can stop them would have been nice. One can figure this out, but it takes a fair amount of flipping back and forth through different sections of the book.
That being said, this might be considered Keep on the Borderlands on steroids. There are all kinds of lairs, ranging from multi-level complexes inhabited by humanoids and giants, to smaller ones filled with assorted critters and undead. There are a fair amount of traps and tricks, but not as many as in Barrowmaze. Descriptions are minimal, but most of the dungeon areas have themes that provide enough mood for a skilled DM to supply details. There are factions that might be played against each other and a few NPCs that might join the party. There is enough variation that I can see my group enjoying this for months. The monsters range in strength from fairly weak humanoids like kobolds up to giants. Still, I would recommend starting this with characters of at least 3rd-4th level since there is a lot of combat involved. In terms of treasure, I'd say that it doesn't offer as much in the way of magic as Barrowmaze, but there's lots of coins and gems. And the gems are always identified by type, size, and quality for people who might be into that. So if your game uses treasure xp, then they can come back frequently, returning to their home base to level up. I'd say that one could easily gain 5 levels or more (starting at 3rd) by completing this adventure.
The larger setting is okay. There is a fairly generic town, briefly detailed, and descriptions of deities and humanoid hordes. The best part about it is that it can be seamlessly dropped into your own campaign world. I've already identified spots for this adventure and Barrowmaze in my campaign world though we haven't actually played either module yet. The new gods are interesting enough and the inclusion of some of their followers helps tie the different dungeons together. There are lots of new spells, magic items, some new monsters, and optional rules for magic, etc. Some of these are better than others, but on the whole, they are pretty good. Most, but not all, monster and magic item stats are included herein. A few are from Barrowmaze. This may irk some players, but both are great dungeons and I don't mind buying them both, even at this high price point for the hardcovers. The maps are pretty good overall, though the hex ones can be difficult to follow as they cover a lot of terrain and are spread over many pages. There are lots of useful illustrations; many gathered together towards the back of the book. I'd have preferred them right in the appropriate text area for ease of play, but this is by no means a deal breaker. There are plenty of random encounter tables, including encounters that are purely atmospheric, which is great. Pre-generated characters and rival adventuring parties are also provided. There are a few too many typos, but fewer than one finds in many of these publications nowadays.
Overall this is a very good product. It offers plenty of action, some problem solving, and a plethora of foes in true old school fashion. It can be easily used with any of the standard OSR rule sets or 1st/2nd edition AD&D, though it is technically written for Labyrinth Lord. Highly recommended if you like killing bad guys and taking their stuff!