"Open" & "Guarded" (to attack)

[YeOldeDM]

These two terms describe whether a an attack against this creature is at Advantage (Open) or Disadvantage(Guarded). These could be thought of (and treated as) status effects.
The rules regarding these states should remain simple. Usually a creature is either Guarded or Open (or neither) to all attack. Special cases will exists, but make them rare and Try to avoid weird exceptions.
Along the lines of "Guarded against melee and Open against Ranged" should be about as deep as it goes.