A dungeon is a group of rooms connected by corridors. It can be a cave, an abandoned mine, or a crypt. It can be an ancient temple, the basement of a stormy castle, or anything you can imagine - as long as it is filled with monsters and treasure!
(from Dungeons & Dragons Game Rule Book, 1991 TSR Inc.)
The most familiar setting of the role-playing games that focus on hack-and-slash...
The words above from the rule book, and the similar words in the earlier editions of the game, have undoubtedly meant a lot to role-playing games and other fantasy-themed games.
What lies in the room ahead? Is it empty? Does it have some devilish traps? Are there monsters in there? Perhaps even a
liar lair of those monsters? Or something completely different? Maybe treasures?
In the flicker of your torch, you look around. You've seen rooms like that many times before. You have placed a heavy treasure chest on your back and you, and your friends, walked home with that box of gold to mourn your dead comrades that perished on your journey. In the same flicker, you have seen strange magic, glitter of priceless jewels, strange scrolls with arcane languages written on them, and strange stuff that was invented by some long-forgotten person in Lake Geneva, WI.
This is your character's life, and it's ending one game turn at a time.
You're not your 2-handed sword.
You're not your plate mail.
You're not your fucking
Dungeons & Dragons was set, as the name suggests, in the Dungeons. Or, at least it always started from the dungeons - as the characters gained more experience and fame, they started adventuring also in outdoors, and finally settling down and starting to rule their own little realms of influence. The dungeons - the dark, scary, treasure-filled hellholes somewhere in dangerous places in middle of nowhere - were the springboard to the fame. Every adventurer, early in the career, goes in to the Dungeon to find some gold to buy something to fill stomach with, and some XP to get better stats.
The Dungeon was always an easy way to get the players used to basic game rules, such as combat - and the rest of the part of the rules that did not actually deal with "roleplaying". Learning the actual roleplaying could always wait until the second game... early in game it'd be more important to learn how to create a character and how to fight. You and your friends - the other characters - were just going for a stroll to a forgotten cave and bring back the rumored treasure that is guarded by some mindless monsters. What could be easier than that?
Not surprisingly, the Dungeons became the greatest thing that ever happened in computer RPGs. Since "go to the dungeon and bring back treasure" isn't too complicated to implement without an actual game master, it has always been the big favorite of the CRPG designers. And over the years, these people have done dungeon romps with fairly amazing amount of depth - Nethack still remains my favorite. While the plot can be condensed to "go down there, bring back an amulet and go up to sacrifice it to your god", the game is not too easy and still keeps in its grip. Random dungeons have always been fun. D&D has rules for making randomized dungeons, and some even say that many D&D adventure modules were created this way... many computer games, most notably the Roguelike games like Nethack, also utilise random dungeons. Admittedly, random dungeons are more on their element in computer games that don't even pretend to have a complex plot...
But beyond randomized dungeons, there's the careful design. Once you give more sophisticated goals, add devilish traps, and add unforgettable goals and non-player characters, it no longer feels like a "dungeon", even when it superficially seems like one. With a touch of thought, the Dungeon gets a soul. At first, you was banging heads in some forgettable location. Then, you got to bang heads in a calm and orderly fashion. And now, you suddently need to think which heads you should bang and when in order not to get your own head smashed. Slowly, the dumb barbarians become... talkative. The setting, the dark corridors, didn't change, but suddently, you have to think. First, it was enough to cooperate with other players and their characters, and then, you also needed to cooperate with other characters you met. First steps from the hack and slash to real roleplaying, and you didn't even need to get out of the Dungeon!
So, dear adventurer: Sooner or later, you will end up in the Dungeon of one form or another. You may not choose when, but when you get there, you can expect to return as a hero if you do your job well. It is an experience each of you need to do once. And why not? It's easy money!