Rules
A Siege's controller can't be its protector. If a Siege's protector ever gains control of it, they choose a new player to be its protector. This is a state-based action.
A battle can be attacked by all players other than its protector. Notably, this means a Siege's controller can attack it.
A battle can be dealt damage and be target of spells and/or abilities that target "any target."
A battle's "defense" is displayed in the bottom right corner of the card. A battle enters the battlefield with that number of defense counters. If another permanent enters the battlefield as a copy of a battle, it also enters with that number of defense counters.
As a Siege enters the battlefield, its controller chooses an opponent to be its protector.
Battles can't attack or block, even if one also becomes a creature. If an attacking or blocking creature somehow becomes a battle in addition to being a creature, it is removed from combat.
Damage dealt to a battle causes that many defense counters to be removed from it.
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.
If a battle has no defense counters, and it isn't the source of a triggered ability that has triggered but not yet left the stack, that battle is put into its owner's graveyard. This is a state-based action. This doesn't cause a Siege's intrinsic triggered ability to trigger.
If a battle that's being attacked somehow stops being a battle, it is removed from combat. Similarly, if its controller changes in the middle of combat, it is removed from combat.
If a non-battle permanent that is already on the battlefield become a copy of a Siege, its controller chooses one of their opponents to be that battle's protector. However, it will most likely be put into its owner's graveyard because it has no defense counters (see below).
If a permanent that is represented by a transforming double-faced card becomes a copy of a Siege, it will be exiled as that Siege's triggered ability resolves, then it will be cast transformed. Note that this applies only to transforming double-faced cards, not to modal double-faced cards that can normally be played using either face.
If a token or a card that isn't represented by a transforming double-faced card becomes a copy of a Siege, it can't be cast as its triggered ability resolves. It will remain in exile. If it's a token, it will cease to exist the next time state-based actions are performed.
If you sacrifice Caetus while casting a noncreature spell (say, to activate a mana ability), the spell won't have convoke when you pay its costs unless it has convoke some other way.
In a multiplayer game, if the protector of a battle leaves the game and that battle is not currently being attacked, its controller chooses a new protector for it as a state-based action. If it is being attacked, its controller chooses a new protector for it once no creatures are attacking it. This means that it continues to be attacked and can be dealt combat damage as normal.
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.
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."
Because convoke isn't an alternative cost, it can be used in conjunction with alternative costs.
If a creature you control has a mana ability with {T} in the cost, activating that ability while casting a spell with convoke will result in the creature being tapped before you pay the spell's costs. You won't be able to tap it again for convoke. Similarly, if you sacrifice a creature to activate a mana ability while casting a spell with convoke, that creature won't be on the battlefield when you pay the spell's costs, so you won't be able to tap it for convoke.
Tapping a multicolored creature using convoke will pay for {1} or one mana of your choice of any of that creature's colors.
Tapping an untapped creature that's attacking or blocking to convoke a spell won't cause that creature to stop attacking or blocking.
When calculating a spell's total cost, include any alternative costs, additional costs, or anything else that increases or reduces the cost to cast the spell. Convoke applies after the total cost is calculated. Convoke doesn't change a spell's mana cost or mana value.
You can tap any untapped creature you control to convoke a spell, even one you haven't controlled continuously since the beginning of your most recent turn.
or one mana of that creature's color.)
At the beginning of your end step, untap up to four target creatures. 
