In these last years the growing availability of personal mobile devices has boosted the demand for mobile services offered based on location and context. Mobile gaming services are, of course, no exception. Unfortunately, the 3G networks infrastructure struggles to keep pace due to scalability limitations and a pull-based service model. One feasible solution, also for gaming, is to adopt an Opportunistic Network (ON) to exploit unplanned contact opportunities between players to perform data exchange and forwarding in ad hoc mode. When designing a game for an ON, the main issue is about forecasting the feasibility of a given gameplay when applied to a real - and complex - environment. In this paper we address this issue proposing an opportunistic game that is meaningful for the category of collaborative puzzle games. The game is evaluated by means of simulations on the topology of a real city in order to understand its scalability and setback on the gameplay.