In this design-based research project, we investigated how and to what extent participating in architectural building and problem solving via a simulation game-based learning platform (called E-Rebuild) would facilitate mathematical practices of middle-school students in the informal learning setting. This study encompasses 2 iterative design experiments in which 20 middle graders completed E-Rebuild-based learning tasks as part of an afterschool program and a summer math camp. Data were collected via screen- and webcam-recorded participation actions and reactions, mathematical problem-solving test, and interviewing. Enriching prior research on math and science learning through making, this study informs how and under what contexts mathematical practices and disposition emerge during the making-centered learning processes.