If a Siege never had defense counters on it (perhaps because a permanent became a copy of one), it can’t have its last defense counter removed. It will be put into its owner’s graveyard. You won’t exile it or cast the other face.
Invasion of Karsus // Refraction Elemental
Sieges each have an intrinsic triggered ability. That ability is “When the last defense counter is removed from this permanent, exile it, then you may cast it transformed without paying its mana cost.”
Only creatures controlled by a battle’s protector can block creatures that are attacking that battle. This means a Siege’s controller can never assign creatures to block for it.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Zoltan Boros
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