(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When this Siege enters, create two 1/1 blue Kraken creature tokens with trample.
Noncreature spells you cast have convoke. (Your creatures can help cast those spells. Each creature you tap while casting a noncreature spell pays for or one mana of that creature's color.)
At the beginning of your end step, untap up to four target creatures.
Flip Invasion of Segovia into Caetus to make your creatures double as a mana engine, convoking out a flood of noncreature spells (counters, card draw, big finishers) while untapping up to four creatures each end step. You go wide with tokens, then leverage convoke to cast spells far ahead of curve and slam control over the late game.
Convoke on every noncreature spell turns a wide board into explosive ramp for counterspells, draw, and bombs
End-step untap keeps blockers up and refuels your convoke pool for the next turn
Comes down cheap as a 3-mana Siege that immediately makes two trampling Krakens
Mono-blue gives access to the format's best card advantage and counter magic
The Battle must be defeated before you get Caetus, and opponents can choose not to attack it, slowing your plan
Board wipes gut your convoke engine and leave you with no mana army
Mono-blue struggles with permanent-based removal for resolved threats and enchantments
Caetus himself is a vanilla 6/6 with no evasion or protection once flipped
Caetus, Sea Tyrant of Segovia
The payoff: convoke on noncreature spells plus untapping four creatures each turn is the deck's whole engine.
Add more cheap, recurring token makers (Talrand, Murmuring Mystic, Young Pyromancer-style blue analogs, Brudiclad) so you always have convoke fodder, and load up on impactful noncreature spells that benefit most from being effectively free. Layer in untap synergies and mana doublers to abuse the end-step untap, and lean into a clean wincon like a big X-spell or an infinite-mana outlet to convert the engine into a finish.
Overwhelm with a wide Kraken/token board after convoke clears the path
Resolve a game-ending finisher like a huge X spell or Stroke of Genius mill
Battle — Siege // Legendary Creature — Serpent
or one mana of that creature's color.)