Help understanding Caelum cold armor bonus

[Saxon]

Taking it a step further, start building temples early to push your dominion out. You want everywhere you fight to be cold. Then, depending on your long term plan, consider some of the global spells that take effect in your dominion. Wrath of God is fun! Gift of Health would go nicely with your units with extra ice armor and the morale bonus that comes with fighting inside your dominion.