Game based-learning have been widely promoted to overcome the difficulties encountered by beginners to learn programming. However, there are many issues to address for the implementation of game-based learning. Indeed, game-based learning is not limited to adding game elements such as rewards to a learning situation, but it rather consists of transforming the learning situation so that it becomes playful. This work contributes to computer science education research, especially to game design for learning programming. We design a novel environment dedicated to learning object-oriented programming for beginners called Progo. It is based on a metaphor of a three-dimensional (3D) construction and animation game. We present an "a priori" analysis of the Progo environment on the basis of a didactic transposition framework. The framework highlights the ludicisation and metaphorisation process by which educational content is integrated into the game. This allows for the analysis of the transformation of the computing knowledge by the game design, and to verify whether analogies are maintained between the knowledge and what the learner should experience through play. This work contributes to a framework for the integration of educational content during learning game design.