Also, some monsters don't have armor at the base card level but gain it when they are leveled up.
Armor can be debuffed with the Rust ability.
Protect and Rust are opposite abilities. Protect adds 2 to Armor, and Rust subtracts 2 from it or removes it completely if the armor level is 2 or lower. Protect and Rust are also stackable, meaning they have cumulative effects.
Void Armor ability or Weak Magic ruleset makes Armor effective against Magic attackers too.