This solution will build Assembly-CSharp.dll to let you connect to servers.
Replace in location:
Leviathan Warships\Leviathan_Data\Managed\Assembly-CSharp.dll
To debug, use 32-bit dnSpy with Unity 4.0.0 mono. The game was created in Unity 3.5.7, but I do not have the debugging mono.dll for this version.
The modified mono.dll file for debugging purposes can be found in the Debugging folder. Keep in mind that debugging is not bug-free in dnSpy.