30% of the project written in windows-only C#.

Game: 

How can it be compiled on Linux?

http://www.ohloh.net/p/Zero-K/analyses/latest

Forums: 

I don't know programming. But it looks the game don't need to be compiled, XML and lua define all rules running on springrts engine.

C# part is mainly server side and editing tools. And also the most popular SpringRTS lobby zero-k lobby, which was compatible with Mono but later dropped due to no Linux maintainers.

>How can it be compiled on Linux?

You don't need to build it because it's a Spring-Mod.