TNet server is a mono application that doesn't require Unity. You can run it from within Unity, or you can launch it as a stand-alone executable (TNServer.exe). You can compile it yourself (TNetServer.zip), and modify it to suit your needs. You don't need Unity in order to run it.
To run the client, you do need Unity as it uses it extensively. The core of the client can be separated, and does not require Unity -- but there are no provided examples of how one would separate it, which is why I said "meant to be used with Unity".