This article presents the underlying philosophy of Sustain City, an educational serious game system that engages students, particularly prospective and beginning science and engineering students, in a series of engineering design challenges. Various strategies implemented in Sustain City for achieving a balance of fun and learning are discussed, including narrative-learning synthesis, supplementing the player's actions with feedback, and the development of a sufficient guidance system. The evaluation of Sustain City deployment is also presented. The assessment confirms the values of the serious games in promoting students' interests and learning in STEM fields.