I first heard of this little "framework/engine/template/starterkit"-thing some months ago - ScriptTD.
ScriptTD is basically a template for a Tower Defense Game for Windows Phone, that you can change (pretty much actually) just by editing XML files.
The project is done in XNA and I have played around with it little now and it's really great. It's amazing how much you can change and how different you can make the game just by editing XML. ScriptTD is open source, so if you know XNA and want to make a Tower Defense Game, ScriptTD should be your starting point.
I just submittet a small game I made (with help from family members for sounds and graphics). The game is called "Frost Defense" and will be in the Marketplace in the coming weeks (I hope :) I'll write about it when it's ready to download).
So, if you want to make a Tower Defense Game for Windows Phone - you should take a look at ScriptTD at Codeplex.com.