@djabwana
You can restrict the movement of units by step and switch them on & off, between steps, not just by turn as in 1888A Steam & Steel
eg (I have not played your game, so this will probably be wrong)
turn off all movement for none Battleships/Cruisers, Heavy Artillery, and Railguns
step Move Battleships/Cruisers, Heavy Artillery, and Railguns
turn on all movement for none Battleships/Cruisers, Heavy Artillery, and Railguns
Search go winter in the xml and apply the principles to different step phases of a turn
https://forums.triplea-game.org/topic/4056/1888a-steam-steel-official-thread
Here is an overview, there are a lot of components, you will not them all
Winter
Search
go winter in the xml to see the components that make up how to use terrain effects and a dummy player
needs an allied dummy player, eg.
winterp
define TT & SZ to be affected
define a new allied relationship eg
winterr allows combat takes over in TT/SZ (oops has no go winter)
needs a WinterLand & WinterSea to be shown in the TT & SZ
define territoryEffect eg. Winter
Triggers to make it happen when you want it. Note -reset- has to redefine the TT/SZ when a TT/SZ changes effect
Hope this helps