http://aushestov.ru/challenges-for-game-designers-%d0%b3%d0%bb%d0%b0%d0%b2%d0%b0-3-%d0%b4%d0%b8%d0%b7%d0%b0%d0%b9%d0%bd-%d0%b3%d0%be%d0%bb%d0%be%d0%b2%d0%be%d0%bb%d0%be%d0%bc%d0%be%d0%ba/
Challenges for Game Designers: Глава 3. Дизайн головоломок