Weland is a Marathon map editor by Gregory Smith, written in C#, and
using GTK#
It is available under the GNU General Public License, see the file
COPYING for details.
To run it, you will need a C# runtime (either .NET 4 or Mono 2.10 or
higher), as well as GTK#
Windows
Make sure you have the .NET runtime installed
http://msdn.microsoft.com/en-us/netframework/default.aspx
Install the GTK# for .NET package
http://www.go-mono.com/mono-downloads/download.html
Mac OS X
Download the Mono runtime, which includes everything you need:
http://www.go-mono.com/mono-downloads/download.html
Linux
Install Mono and gtk-sharp from your distribution's package system