Confidence:StrongSolidPlausibleTentativeSpeculative

• Mechanized Production

  2UU

  Enchantment — AuraU

  Alternate win condition: put the wincon on ONE of the tokens the other card produces (tokens of a batch share a name — Servos, Thopters, Treasures), and every further copy counts toward the same-name total.
• Esper Sojourners

  WUB

  Artifact Creature — Vedalken WizardBUW

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Fiery Gambit

  2R

  SorceryR

  Fiery Gambit untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Innocuous Researcher

  3G

  Creature — Centaur DetectiveG

  Innocuous Researcher untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Magus of the Candelabra

  G

  Creature — Human WizardG

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Candelabra of Tawnos

  1

  ArtifactC

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Beledros Witherbloom

  5BG

  Legendary Creature — Elder DragonBG

  Beledros Witherbloom untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Formidable Speaker

  2G

  Creature — Elf DruidG

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Saryth, the Viper's Fang

  2GG

  Legendary Creature — Human WarlockG

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Prophet of Kruphix

  3GU

  Creature — Human WizardGU

  Prophet of Kruphix untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Dragon Man, Reformed Robot

  2WU

  Legendary Artifact Creature — Dragon RobotUW

  Recursion engine: one card can reanimate repeatedly (an activated ability, or a return that comes back once a turn), and the other is a creature worth bringing back again and again — or a creature you recast from your graveyard beside a sacrifice outlet that doesn't pay for the recast. Real, repeatable value — not an infinite loop on its own.
• Bumi, Unleashed

  3RG

  Legendary Creature — Human Noble AllyGR

  Bumi, Unleashed untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Mysterio, Master of Illusion

  3U

  Legendary Creature — Human VillainU

  Both cards produce the same kind of effect (shared token subtype).
• Retrieve the Esper

  3U

  SorceryU

  Both cards produce the same kind of effect (shared token subtype).
• Wilderness Reclamation

  3G

  EnchantmentG

  Wilderness Reclamation untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Seedborn Muse

  3GG

  Creature — SpiritG

  Seedborn Muse untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Crimson Cowl, Master of Evil

  2B

  Legendary Creature — Human VillainB

  Both cards produce the same kind of effect (shared token subtype).
• Sword of Feast and Famine

  3

  Artifact — EquipmentC

  Sword of Feast and Famine untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Kavaron Harrier

  R

  Artifact Creature — Robot SoldierR

  Both cards produce the same kind of effect (shared token subtype).
• Madame Hydra

  2BR

  Legendary Creature — Human VillainBR

  Both cards produce the same kind of effect (shared token subtype).
• Criminal Enterprise

  2B

  EnchantmentB

  Both cards produce the same kind of effect (shared token subtype).
• Nature's Will

  2GG

  EnchantmentG

  Nature's Will untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Emrakul, the Exigent Doom

  10

  Legendary Creature — EldraziC

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.
• Awakening

  2GG

  EnchantmentG

  Awakening untaps all (or many) permanents, refreshing Castle Doom's tap activation.
• Captain of the Mists

  2U

  Creature — Human WizardU

  One-directional refresh: one card can untap permanents the other wants to reuse. Not self-sustaining on its own — the untapper has to keep showing up.