To complete the "Pandora21" project, that tackles the issue of creating awareness and action concerning sustainability by using "serious games", we propose a methodological and technical solution to make the space of the game massively "participative", at the scale of a city or of a territory. Such a space has to propose rich functions allowing not only to confront multiple players using the existing objects of this space, but also to co-design the space of the game. Co building of the scenes, discussing of the rules are possible for a wide group. Local partners of a given city will be able to add easily scenes, elements of scenes and micro-games transposing appropriate sustainability situations for their city by rapid prototyping, without having to pass by the computer specialists. Hypertopic, a multi-viewpoints model, will used to ensure a plurality of views on the city. The space of the game will be indeed "participative" at the same time for players and designers groups.