We present a wizard of Oz framework for the simulation of locationbased mobile games. We explore briefly the methodological concept of Wizard of Oz and show, based on a case study, advantages and shortcomings of the method. The case study realized a game called capture-the-flag. We show how such prototypes can be evaluated in a real field trial. Categories and Subject Descriptors H.5.2 [Information Interfaces and Presentation]: User Interfaces - evaluation/methodology General Terms: Human Factors.