Rule 310.12 · Battles
Some battles have the subtype Siege. Sieges are subject to special rules.
Subrules
As a Siege enters the battlefield, its controller must choose its protector from among their opponents. Only an opponent of a Siege’s controller can be its protector.
Sieges have the intrinsic ability “When the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost.”