FUNDAMENTALS OF GAME DESIGN, SECOND EDITION

. Simultaneous Atomic Challenges

Although the player simultaneously faces several challenges in the hierarchy, she pays less attention, on a moment-by-moment basis, to those challenges that are far­ther up in the hierarchy. But games can also present several atomic challenges at once. These divide the player's attention. If she can deal with them one at a time
at her own pace (as in a turn-based game), then they're not really different from sequential challenges, but if she has to surmount them all in a limited amount of time, then adding simultaneous challenges makes the game more stressful. (Stress is discussed in the next section, "Skill, Stress, and Absolute Difficulty.")

An early and still common way of creating simultaneous atomic challenges, typical of side-scrolling shooter games, consisted of bombarding the player with enemies. Each enemy represents a significant risk, and the player must defeat each one while fending off the others. A player who works quickly can generally defeat these added enemies one at a time while keeping the others at bay.

Other games present more complex and interrelated simultaneous challenges. In its default mode, SimCity imposes no victory condition; its highest-level challenge is to achieve economic growth so the player can expand his city. (Expanding the city itself isn't a challenge, just a series of choices available so long as the player brings in enough money to keep going.) The player doesn't attain economic growth unless he can provide a balanced supply of services to the city. The city needs police pro­tection and power and hospitals and water and so on, all at the same time; each represents an atomic challenge, and the player must meet all of
these simultaneously. The complex juggling of competing needs requires regular attention and frequent action. Furthermore, unlike fighting enemies, the player can never finish balancing the services; the juggling act never stops.

It's part of your job to design the hierarchy of challenges and decide how many of them the player will face at once: both vertically up the hierarchy and at the bot­tom of the hierarchy. The more simultaneous atomic challenges he will face under time pressure, the more stressful the game will be. The more different levels of challenge he will have to think about at once—especially if he can't simply achieve the higher ones by addressing the lower ones in sequence—the more complex and mentally challenging the game will be.