Looking at the files I`m guessing a new line needs to be entered in terrain.pzdat and that needs a corresponding line in strings.pzdat. The image for the editor needs to go in terrain.png from Graphics/Editor and then each movement type from movement.pzdat should get a new entry for the new terrain. That said, how will the engine know what tile background and overlays to place on the designated hex in the editor when it generates the terrain? I feel I`m missing something.
