What are some alternatives?
When comparing PlayN and Castle Game Engine, you can also consider the following products
Unity - The multiplatform game creation tools for everyone.
Pygame - Pygame is a set of Python modules designed for writing games.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
CryENGINE - The most powerful game development platform is now available to everyone. Full engine source code.
Torque 3D - Torque 3D is a easy-to-use game creation platform.
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.