What are some alternatives?
When comparing RenPy and Castle Game Engine, you can also consider the following products
GDevelop - GDevelop is an open-source game making software designed to be used by everyone.
Unity - The multiplatform game creation tools for everyone.
Unreal Engine - Unreal Engine 4 is a suite of integrated tools for game developers to design and build games, simulations, and visualizations.
Stencyl - Make iOS (iPhone/iPad), Android, Flash, Windows & Mac games without code using Stencyl.
Torque 3D - Torque 3D is a easy-to-use game creation platform.
BYOND - BYOND is the premier community for making and playing online multiplayer games.