At the heart of the world is the City, built atop the mysterious Dungeon. There is no need for any other names for either, as they are the only things in the world that matter. The Dungeon is a vast labyrinth filled with infinite treasure and magical power. Merely entering the Dungeon requires becoming immortal, and dedicating your entire life to forever delving deeper and deeper.
But the Dungeon is far from harmless. It is filled with monsters and traps, as well as human opponents who will slay fellow delvers and steal their immortal heartstones. Treasure and power are infinite, but so is danger. And the deeper you go, the smarter the Dungeon's dangers become. Almost as if the Dungeon itself is intelligent, and hates all who delve within it.
This novel provides examples of:
- Adventure-Friendly World: Deconstructed twice over. First, failure rates for delvers are ridiculously high, especially new delvers. Second, if a delver does succeed in gaining power and money, they quickly become more powerful than either the City or the outside world can handle. Any Blue-spectrum delver or above can crash the economy, fight armies, and altogether do absolutely whatever they want unless another, stronger delver tries to stop them.
- After the End: Something happened before (or right around the same time as) the Dungeon was discovered. Mention is made of "Earlier" cities that have been destroyed, and ruined cities still covered in the claw marks of massive monsters.
- And I Must Scream: Being killed and reduced to a heartstone is complete sensory deprivation, with only your thoughts for company. Worse, once you are revived you can't actually remember what you thought while you were dead; it's described as an extremely disturbing situation. Being shattered is apparently even worse, as you remain aware of each and every shard. No one knows what it feels like to be shardgear, because no one has managed to revive anyone from that state yet.
- Cap: Stats max out at 1000; a few high-tier classes automatically max one or more stats when you take the class. Classes, on the other hand, have a soft cap of 100; you can keep leveling after that, but the experience requirement is exponential, so it's considered a waste.
- Complete Immortality: In theory, it is impossible to permanently kill a delver. On death their heartstone can be recovered and revived, and even if their heartstone is shattered (something which can happen in the Deep or below), they can eventually be reassembled and revived. However, they need to eat crystal to survive, and the more powerful they are, the more crystal they need. If they "starve" they just revert to crystal again and can be revived, but they probably have no money, so no one will bother reviving them.
- Damn You, Muscle Memory!: Alex nearly kills his sister when she slaps him; after a year of fighting in the Dungeon, his reflexes are uncompromising. Thankfully, he stops himself just in time.
- Fake Difficulty: The Core has the exact same monsters as the first twenty-five floors of the Dungeon, with the same cheap gear that even low-level parties ignore. Except these monsters have the "Core" modifier, boosting their stats to ridiculously high levels. Some of the most powerful delvers in the City get killed by one goblin with a knife that's not even worth a red crystal.
- Fanservice with a Smile: Since it's possible to increase Charisma with potions, most people in the service industry are very attractive. Apparently there's a section of the Law that limits the Charisma outdoor waitresses can have, to prevent a "cuteness arms race."
- Fantastic Naming Convention: Some delvers take the name of the first monster that successfully killed them. T.B. Kyoto, for example, was killed by a Tengu Bushi.
- Fiction 500: All the upper-spectrum delvers. One violet delver randomly decides to drop a blue (less than pocket change to her) to buy all the clothes and clothes shops in the City, and absolutely no one can stop her. When Alex returns home after going violet, he idly notes that he could buy his entire birth city without any difficulty at all.
- Get a Hold of Yourself, Man!: Alex's sister slaps him when he's rambling how he'll never be able to ask out Alice Black. He nearly kills her before he stops himself.
- Genius Loci: Some delvers believe that the Dungeon is intelligent, and will spawn more monsters or treasure under certain circumstances. Notably, very few people believe that the Dungeon is benevolent, even when they seem to be having an improbable amount of good luck. When the superparty discovers that lost heartstone shards are forged into incredibly powerful weapons and items—while the person is still alive—everyone silently agrees that the Dungeon is unquestionably evil.
- Ghost Memory: Using a skill from a skillstone feels like reliving memories you've never had; you have a vague recollection of past mistakes and how to avoid them. Spellstones don't have the same feeling, which Xavier theorizes is because no one outside the Dungeon has ever cast a spell, so spellstones are only giving the most basic knowledge.
- Heroic Neutral: The City has a strict non-intervention policy with the rest of the world, and a delver harming a surfacer under any circumstances is heavily penalized by the Law. This is because delvers are ridiculously overpowered compared to the rest of the world; a single violet could probably conquer the world by themselves, but even a blue can cripple the world's economy without too much trouble.
- Jerkass Gods: The Eidolons have special gear that turns anyone, regardless of power, into a violet-level special class that mimics the Greek gods and monsters. They're all assholes (except Hermes), and if one of them dies they collect the gear and abandon the now-powerless normal person while they find someone else to elevate to godhood. Notably, the Eidolon Arachne is the one who went around buying all the clothing in the City for no apparent reason.
- Inherent in the System: Delvers need crystal to survive. The deeper they delve, the more experience they gain, the more crystal they need, and the deeper they need to delve. It's very difficult for a delver to just retire. While the City is more mutable, the High Houses have also constructed the system to encourage delving deeper and deeper as much as possible. Late in the novel, they start ordering all their contracted shops (which is to say, basically all of them) to charge more to low-level delvers to force them past the first few floors.
- Not the Intended Use: One of the things the City trades to the outside world is pieces of the Dungeon that still function. Rich people often use Dungeon doors with an alarm trap on them as doorbells. Delvers have a strong dislike for this practice, as they have a deep and instinctive distrust for all traps.
- Physical God: All upper-spectrum delvers. Special mention goes to the Eidolons, who have special gear that turns ordinary delvers into violet-level gods modeled after the Greek gods and monsters.
- Vancian Magic: V-type magic works this way. It's noted to be less flexible than M-type magic, but generally more powerful.
- Worthless Yellow Rocks: Due to the way crystals work, this is inevitable. Red crystals are roughly equivalent to the currency surfacer countries use, but orange crystals are worth a thousand times more, yellow a thousand times more than that, so on and so on. When Alex first becomes blue he tries to send a blue crystal to his family, and is informed that this one crystal could kill the economy of his entire home country. He sends yellows instead, a million times less than what he wanted, and when he returns home he finds that his family is now the richest in the city by a very large margin.