Role-Playing Games are dungeons.
Sure, there are a lot of other aspects to Role-Playing Games, but their heart is the dungeon. Most games have only a few other states you can be in - traveling, in town, and editing your character. And most would have you spend as little time in those other states as possible.
The three most classic dungeon locales are the ruined keep, the cave system, and the city sewers. But any locale that inherently is all about confronting obstacles, challenges, and enemies is a dungeon. You can have a dungeon on catwalks in the sky, if you like. Or an abandoned space station at the edge of the universe. Or in an underwater grotto.
Dungeons are everywhere all across the spectrum of Role-Playing Games. And since they are everywhere, it is best that the dungeons you use in your campaigns be great ones.
Yet, it is more than that. As just stated, dungeons are the heart of Role-Playing Games. To extend that metaphor a little bit - the state of In Town is the mind of the RPG, and the Character Sheet is the soul. You can lose any one of those things, and it will still be a Role-Playing Game, but without character and the character sheet, the game will be empty. Without In Town, there will be no intelligence and no social dilemmas, no wit, no cerebral experiences challenging the Players' beliefs and mores.
And without the Dungeon, the game will have no pulse.
Within the Dungeon lies all of the physical and mental challenges of the game. In other words, all of the action. This is where the games gets all of its thrills and chills. And so, if you, as Gamemaster, are planning on telling tales of action and danger, then you need the Dungeon. And if those Dungeons are going to be the centerpiece of your campaign - as they most likely will be - then they must be great Dungeons for your game to be a great one.
You already know a lot about how to make a Dungeon great (from the first book in this series). Within these pages, you will find additional tools that you can use to ensure that every Dungeon you make will be a great one.