How do I prevent units from moving ? Not just a specific unit for a specific player. Must all units. I've seen it done before. I think it will help with some ideas I've had.

If I'm not mistaken, by making a location "outside" of the map boundaries, and constantly move all units for all players to that location.

Awesome! Thank you.

Does it matter where? Top left? Bottom right? How far in does it have to go? I will try and play around with it later.

Actually it's enough to move it anywhere where the unit cannot go. This could be a (fairly large) occupied air space for air units, water/cliff for ground units or just somewhere outside the map.

Really doesn't matter where, as long as the unit can't actually be moved there.