This paper presents the Rallye Platform, a software ecosystem that enables various mobile location-based serious games such as digital treasure hunts. Such games are an effective way to engage audiences in a cultural heritage context on-site, and beyond traditional museum spaces. We devised and employed the platform to develop two longrunning deployments -the Légionnaires Rallye and the Minett Stories Rallye-in support of two major multimedia exhibitions in Luxembourg running between July 2021 and May 2022, which doubled as test cases for qualitative and quantitative analysis. The evaluation shows that the applications helped promote the physical exhibitions, engaged players, and sparked curiosity about the respective subject areas and historical contexts. Various changes implemented between the deployments further improved the user experience, and the mobile web applications were used even beyond the end of the exhibitions by students and other groups to explore the history and geography of the urban space.