The Game Space
The Game World design pattern has some great information about Game Space design and modulated links are also worth exploring.
Games occur in a physical space. The space defines the playing field both in determining how the game is played and the feel of the game. Open spaces can allow the player to explore and/or use strategy, while linear games have set paths that players must travel in order to progress in the game. Open spaces usually require secrets to be discovered or tasks accomplished sometimes the order of the tasks matters sometimes they don’t.
Obstacles in the game space can be used to allow player to shield or hide from enemies. Barriers and walls can used be used to direct players along certain paths, as can raising or lowering the terrain.
Special Spaces, indicated by colour or texture, can alter game play by changing the rules or effecting game play (eg the player moves slower in snow)
The camera, also affects the game space. A fixed camera above or to the side of the game space creates a 2 dimensional field but is often useful when creating multiplayer games. First person cameras are useful when creating games with mazes and surprises and much of the game space is often unseen by the player.
Levels
A simple way to give a sense of progression is to have levels in your game. It should be clear to the player that a level has been completed and that a new level has started. Typically a player is unable to return to the previous level once the next level has been started. Usually, a Boss Level is encountered in order to complete the game.