5. Overlapping Events and Behavior

Dynamic is lost if only one change occurs at a time. Discover the right amount of events to occur at any given moment of time.

Level design example
Providing the player the ability to build from an appropriate list of structures.

System design example
The linebacker points to direct fellow players, the defensive end shifts over, the quarterback points and calls out football jargon and the crowd cheers louder because it's third down. All this occurs before the snap.

UI example
Points accumulate in the score while each kill is individually tallied on screen.

Story Example
Multiple plot points are at the forefront of the narrative experience. Example: the king is on his deathbed while his war is being waged and he has yet to announce an heir -- all while an unknown saboteur orchestrates a military coup.

6. Physics

The player's primary logic operates within the known possibilities of physics. Keep in mind gravity, weight, mass, density, force, buoyancy, elasticity, etc. Use this as the starting point, but do not be limited by it.

Level design example
Ensuring a hole in the floor is the correct size for the correct purpose. Whether it is part of the path of level progression, or simply for visual aesthetics.

System design example
A spark particle effect occurs when the player's vehicle scrapes the side of the concrete wall.

UI example
The GUI's theme references scrapbook elements. In which case, animated transitions, highlights, etc. follow the physical characteristics of paper.

7. Sound

Ask yourself, "What sound does it make when ________ happens?" "Is the sound appropriate?" "Is the sound necessary?" "Does it benefit the experience or hinder it?" If players close their eyes, the sound alone should still achieve the desired affect.

It's debatable whether this principle should be included since Sound Design can be considered separate from Gameplay Design. I've included it because sound is crucial and can easily be neglected. The more it is considered, the better the experience is for the player.

Level design example
Flies in swamp level make a sound when close to the camera.

System design example
A proximity system where sound effects volume fluctuates depending on distance of game assets.

UI example
Only visually prominent graphics have sound effects attached to them, so as not to muffle the auditory experience.

The next three principles individually touch on other major design components.

Progression

8. Pacing

Keep in mind the desired sense of urgency, the rate in which events occur, the level of concentration required and how often events are being repeated. Spread out the moments of high concentration, mix up the sense of urgency, and change things wherever possible to achieve the proper affect.

Level design example
Create areas for the player to admire the expansive view, versus areas where the player feels claustrophobic.

System design example
Create long, powerful attacks versus short, light attacks.

Environment

9. Spacing

Understand how much space is available both on-screen and in-world, recognize the spatial relationship between elements and take into account the effects of modifying those spaces.

Level design example
Lay out the appropriate amount of space for the appropriate number of enemies to maneuver correctly.

System design example
When an AI character moves through a bottleneck area, walk loops switch to standing idle when the AI character is not moving forward, to show that the character is "waiting" to move through the narrowed space.