From Quake Wiki
start - Introduction is the first level loaded in a New Game where the player chooses both the difficulty level and the episode to play in. The player returns to Introduction beyond the skill gates after completing each level with a new rune in-hand, allowing the player to complete episodes out of order if desired. The end of episode story text does not account for out of sequence completions.
The skill selection is broken up into three hallways, left to right:
- Easy - The leftmost hallway, the player walks a flat path past stained glass on smooth brick over blue water.
- Normal - The center hallway, flanked by bloody gargoyle faces, the player walks over ramped metal that covers muddy swamp water.
- Hard - The right hallway, the ceiling is missing, and the ground separated to reveal lava with lava balls popping out of them. The player must jump over the lava to reach the teleporter under jutting spikes and the moaning gaze of crucified zombies.
The episode select halls are all open, unless playing the shareware where all but the first are blocked off by a barrier. Registered users will see these barriers open as they are approached. The episodes are placed sequentially from left to right, each in a theme according to the episode and its place in the story.
- Episode One - Dimension of the Doomed - The entrance to the first episode is based around the notion of where it all began, the Slipgate Complex, and so the surroundings are the base textures and fluorescent lights.
- Episode Two - The Realm of Black Magic - Torches, blue stone, brown and blue tiles, and a large dragon engraved double door set the medieval castle tone for this episode.
- Episode Three - The Netherworld - Raw metal, rivets, and zig-zagging 90 degree angled floors hang over lava under an exposed sky with glowing runes.
- Episode Four - The Elder World - The Elder World is a different beast altogether, wooden slats for stairs revealing stretched skin lanterns lead the player up to a pool that when entered has a surface bottom, a suspended body of water. Falling through the bottom drops the player into an alcove of ancient white stone and wooden pillars, with steps flanked by crucified zombies leading up to the slipgate. It is the only episode hall with a teleporter to exit, or a secret.
As you complete each episode, the entrance will be blocked by a large stone with runes on them. When you have completed all episodes, the floor between the different halls opens up into a grand and raw cobblestone staircase, winding down into a pit with a slipgate at the bottom and message "Shub-Niggurath awaits you..."
Though there are no official secrets in Introduction, there is access to a fourth difficulty setting, Nightmare. Enter the hall for The Elder World, enter the pool and press your back against the wall immediately behind you, then break through the bottom surface. You will land on a wooden railing over the area, follow the railing to the left to find a short passage leading to a lava teleporter, stepping through it will select Nightmare difficulty. Just before this teleporter is a stretched skin lantern on the wall, if you shoot it you will see the message "The Well of Wishes awaits you in the Crypt of Decay." which is an allusion to a Commander Keen level.