Titanium Xna Game Engine is a Xna 3.1 based game engine written in c#. Complex scenes can be build in just few lines of code. The developer is free to combine TXna features and standard Xna techniques without any limitations.
Please use the svn checkout or update feature to get the latest source and compile from source. The engine is not released frequently but only after major source updates.
Chances are that by downloading a release you will get a rather old copy.