If someone could please help. I stopped playing the game for a long time, Came back to see all the cool upgrades. I had to ask on this forum how to unlock units. Now I have run into a different problem. I know how to upgrade using resources. I now have 2 upgrade tokens (total) for 2 different of my new units. Both are air units. Do I need a min level of certain building buildings to use these tokens? If so I would assume the barracks for infantry, war factory for vehicles, but do not know which if needed would be for air units- hangar or airfield.