This paper presents a hackathon-style experience in which fifteen secondary mathematics teachers have been involved in designing a digital game. The experience is part of a project aiming at identifying guidelines for designing and using digital games in mathematics education. The main assumption of the project is that the collaboration among teachers, software developers, and researchers could contribute to designing a mathematical digital game for educational purposes. This paper will focus on the teachers’ reflection on the design by comparing their work during the hackathon with the five elements of software developers’ design principles: interactivity, immersiveness, adaptive problem solving, feedback, and freedom of exploration. Results provide valuable insights into the design of digital games for learning and mathematics teachers’ professional development concerning their use.