Integrating user-generated content into digital games helps to increase re-usability and to decrease development effort. In terms of learning games, the content creation can be extended to the learning parts of the game as well, e.g., allowing teachers to create custom games for their students. In this work, we propose a method of how to create learning games with arbitrary user-generated learning content. This includes not only different topics of learning content, but also different types of knowledge acquisition. To do this we combine a static game scenario with lightweight HTML5-based mini games. Learning content can be conveniently added through a web-based authoring tool that does not require any programming or game design knowledge. In addition to that, we created a game prototype based on the Unity3D engine that uses a tower defence game setting to integrate the learning content. These mini games are retrieved from the backend service at runtime. The implemented solution already allows for the integration of arbitrary content and can easily extended without altering the game client.