Although DwarfCorp is open source, our license gives us ownership over non-code art assets, characters, trademarks, logos, etc. The source code is under an MIT license, the art assets are under a proprietary license. That means you are free to fork DwarfCorp and use the source code in any way (including in commercial projects). You are also free to submit contributions in the form of pull requests provided that all submitted source code and art assets fall under our license.
Implications:
Make sure at the very least that your source code compiles and runs and more or less follows the formatting/naming conventions in the project.
The pull request should have
Someone from Completely Fair Games will take a look at your pull request and make a decision (possibly with notes for how to edit the submission) on whether it should be merged in.