The tone, genre and setting of the original D&D materials was a "quasi medieval" setting. So you have kings and kingdoms, knights, villages, cities, maps, trade, languages and ships.
As for the trappings of the fantastic you have dragons, monsters, and other creatures hiding in secluded places.
For magic you have magic weapons, inspired by things like Excalibur, and wizards, inspired by legends of Merlin and others.
It's a wide open setting, based only slightly on a historical template from our own earth.
It's original rules and mechanics were based on rules created for tabletop war-gaming, where large masses of men fight against each other. This means you had a lot of men and didn't feel too much loss when any of them died.
Dungeons and Dragons the role playing game is based on the idea if each player controlling only one man. And following their exploits as they grew in power to become a "Super Hero".
Now you have a handful of men versus a large number of enemies and a "general".
IF you were to set both groups on a war-game table and let them attack each other then you would have a very short lived game. The enemies would quickly overwhelm them, killing them in a short period of time.
What the "heroes" needed was a way to encounter the same number of enemies but in small groups, starting with the weakest ones first and increasing their fighting abilities before fighting the next group.
This is what led to the 3 main ideas which drove the D&D game and indeed all role playing games afterward.
The first idea was the dungeon.
Think of the dungeon like a battlefield.
You start at one end and work your way through the enemies cannon fodder, the weakest and easiest to start with. The closer you get to the other "end" the more powerful the enemies become, through the shock troops and the generals body guards till you get to the general himself.
The twisting corridors and small rooms of the dungeon are conducive tot he Heroes Guerrilla style warfare tactics of hitting small targets and getting away quickly.
Indeed the first "dungeon" was the basement tunnels of a fortress.
During a war game scenario a small group of skirmishers were trying to overthrow the by sneaking in through a back door.and through the basements, but the players enjoyed that part of the scenario so much it was decided to expand on it.
The next big thing was to figure out was how was a group of mediocre soldiers ever supposed to defeat an enemy so powerful that they threaten the world? Well if the enemy was somehow able to amass so much poer there must be a way for the heroes to do so as well. And so an "experience point" system was born, so that they could incrementally improve until they too were "superheroes" capable of defeating the enemy general.
But there was still one more problem. The heroes were still too weak. If you consider the pacing of an adventure game it becomes clear that heroes have to kill alot of enemies to succeed in their quest.
IF they walk into a dungeon, kill a couple of guys each and then run off to heal, waiting a couple of weeks for natural healing to do it's work then the enemy would clearly move more guards to that room again before they returned.
The heroes had to find a way to press further into the dungeon in a single outing.
And so magical healing was born, healing potions, healing spells and even Resurrection spells were created. No one wanted to spend years creating a character just to see them killed by some lackey with a lucky strike. For Heroes to act like heroes they needed to be able to run in heal quickly while still in the dungeon if things got really bad and move on a little farther.
The idea that
a: massive underground labyrinths exist everywhere in the world
b: that by simply entering them and killing enough enemies and gaining enough treasure will make you more powerful
c: that if you make a mistake you will be magically healed and able to continue on
are the hallmarks of all fantasy role playing games today.
See part 2 for the Sword and Sorcery version of these 3 things.