This kind of question had probably been answered several times, but I'm not sure how much the latest beta has changed things in terms of editing so I'll ask anyhow:
I've been trying to change the terrain.ruleset. All I've been doing is adjusting the numbers for certain values and occasionally changing what irrigation or mining does, etc.
When I do this and try to run Freeciv, the game doesn't load.
I tried doing the change to the 'Experimental' ruleset and the program loaded, but then failed to start start a game because it expected a certain value in the terrain ruleset.
So! Clearly I need to be careful about something. What do I need to be careful about?