FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Make Atypical Levels Optional
Level designers naturally like to vary the content of their levels, and it is good design practice to make creative use of the game's features or to set your levels in different environments to provide the novelty that players like.
Still, you should not create wildly atypical levels and force the player to play them in order to get through the game. Level designers sometimes create a level filled with only one kind of challenge—an action game level consisting of nothing but platform jumps, say, with no enemies to fight or treasure to find. Others like to take away some of the actions that a player uses routinely on other levels and force her to make do with a limited subset of actions for the duration. Some have created levels that borrow from a different genre entirely: a real-time strategy game level in which both sides control exactly one unit, thus turning the level into a strange sort of action game.
There are two reasons not to make these kinds of levels obligatory. First, it breaks the player's suspension of disbelief to be suddenly confronted with a situation that would never occur according to the rules of the game world as the player has already learned them. Second, it may actually make the game unwinnable for some players. If you create a level filled with only one kind of challenge, then a player who happens to be terrible at that kind of challenge—but who reasonably expected to make it through the game by being good at other kinds of challenges—might not be able to finish the game at all, stymied by one atypical level. And there may be many players who don't find that challenge as exciting as you do, who will find an entire level of it boring.
You shouldn't avoid making atypical levels at all; they can be a lot of fun. But make them optional—hidden levels the player can unlock through excellent play or side missions for extra points.
