I have chosen 20 for ice damage. But the <iTurnDamage> tag is in FeatureInfos, not in TerrainInfos? A guy named Xooll made a mod comp called 'Hardship'. There were promotions in it to alleviate the effects. In attachment, you will find my version of it, I made some adjustments. This should give you ideas. Note that the AI has no clue of it because it's Python. I would not use TerrainDoubleMoves. Me on the contrary I used <iMovement>2</iMovement> (meaning 1 tile at the time on horse for example).