Well currently TW haven't released modding tools or any documentation. People are going through game files, DLLs to get info. So most of the mod work you see will have little to none c# code. You can either wait for people to work things out more or wait TW to release tools whichever happens first.
If you want to add new models, type of soldiers, change name of towns etc. that type of stuff then those can be done through XML files.