This paper analyzes the revenue models of the most popular games of the Tower Defense genre on Google Play. A special look is taken at the quantitative distribution of the app sale model and the free model in terms of quality and download numbers. Additionally, this paper considers the qualitative implementation of the “free†model in the most popular games. First, the usual revenue models of mobile apps will be discussed and then the Tower Defense genre will be explained. Following that, the quantitative distribution of revenue models and an analysis of the most popular apps’ respective revenue models will be addressed. The analysis also identifies and explains two modifications of established revenue models. The most popular revenue model for mobile apps in the Tower Defense genre are in-app purchases. This distinguishes the genre from many other genres and games. A wide range of Tower Defense games utilizes the revenue models app sale and free. It becomes apparent that revenue models for mobile apps must be analyzed and considered specifically for their respective sector, and that no single promising revenue model for apps exists.