Creating a mobile urban tourism storytelling application presents several interactivity challenges on how to convey an engaging multimedia experience on-site. This article describes a methodology for fast prototyping of a multimedia mobile applications dedicated to urban tourism storytelling. The application can be a game that takes advantage of several locationbased technologies, freely available geo-referenced media, and augmented reality for immersive gameplay. The goal is to create serious games for tourism that follow a main narrative but where the story can automatically adapt itself to the current location of the player, assimilate possible detours and allow posterior out-of-location playback. Adaptable stories can use dynamic information from map sources such as points of interest (POI), elevation or virtual buildings. The main focus is for these locationbased storytelling games to create more engagement between the tourists and the urban environment. To explore this concept, an application was designed for the city of Porto: Unlocking Porto. This location-based game with a central, yet adaptable, story engages the player into the main sights following an augmented reality path while playing small games. The article discusses and presents solutions for media acquisition, interactive storytelling, game-design interface and multi-disciplinary coordination for mobile app development.