Welcome . . . to the Legend of the Amethyst Futon!
This is not meant to be a real game. It's barely balanced and horribly unfun. It's also not meant to be a guide to proper game layout - the requirements of a console game are very different to the requirements of modern graphical games.
Instead, it's meant to be a showcase of Dec functionality. The intent is that you can read through the code without too much difficulty and pick up some tricks on how to fully exploit Dec.
You definitely should not consider this the One True Way to design games using Dec. Every game is different and has different requirements. Put these concepts in your toolkit, then use them when appropriate.
The project entry point is in Bootstrap.cs and I recommend starting there.