Key to Addictive Game Design

Schell quotes Designer David Perry's key to addictive games: games should be designed so that players are doing 3 things at a time: exercising a skill, taking risks and working a strategy.

Supercell's Clash Royale is a great example of mastery of this: you exercise skills firstly by learning whichs cards are strong and weak against each other, then where to place warriors and eventually selecting a strong team; the game presents numerous opportunties to task risks and there are many strategies that can be explored.

This is related to Schell's idea about Triangularity In Game Design importance of taking risks.