Pacing is the introduction of conflict and tension, plus what some like to call the "adrenaline rush." This follows closely the Thesis-Antithesis-Synthesis model that we know from stories and films. The tension builds as the player (the thesis) interacts with the antithesis, and it crescendos right before the synthesis, where the reader, watcher or player breathes a sigh of relief. (Or, alternatively, the player may get grim from his failure and restart the level, pretending it never happened.)
Because games are interactive, forcing a certain pace into the level can be difficult. What if the players don’t do what you want them to do? What if they take too much time? What if it’s too easy and unexciting when it’s played slow or too intense if played too fast? There are some things you can do to remedy this without taking all the interactivity out of it.
Time limits add tension that’s immediately perceptible by the player. A time limit can force a player to move more rapidly, or adopt tactics that you want him to use, such as splitting forces to achieve multiple objectives. You can put in an artificial time limit – like a mission clock, a puzzle-solver clock, or a turn time limit. You can institute a realistic time limit into a level, like the time it takes a certain enemy or ally unit to move to its exit point, or the time before enemy reinforcements arrive to overwhelm the player.
Controlling the movement speed or distance a player may traverse in a turn drastically affects game play pacing. While you cannot just arbitrarily change this in your level unless you are doing a puzzle game like
Tetris, there are other ways you can play with speed. Often terrain affects movement speed, such as swampy ground that slows you down, a highway that permits you to speed up, or an obstructed and twisty route that slows your progress. Giving units different movement speeds and/or movement restrictions can slow or speed up the players, if they have to travel with that unit. For example, giving the player a slow, heavy tank will encourage his forces to slow down to the tank’s movement speed, and making the player guard a fast-moving ATV will force him to speed up.
Pacing can also be set by the enemies’ speed. For example, in a POV shooter, the player may have to chase down an elusive ninja who’s trying to escape from a lord’s castle. In a side-scroller, the boss monster can be made faster or slower. Whenever a player has to move and act faster or slower than he’s accustomed to, it builds tension. By using these various methods to manage tension through movement speed, you gain precise control over the pacing in your level.