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The player controlling Solitary Confinement must discard a card every upkeep or else sacrifice the enchantment, and the controlling player has to skip their draw step

Getting the opportunity to play a free accelerating spell is extremely powerful (that will be a common theme with the top of this list) but comes with a card disadvantage-based downside you have to exile a (non-artifact) card permanently to make any mana at all

Between flashback and the ability to discard multiple copies of a card at once, the ceiling is lost in the clouds

I actually think the fact that the format was so bomb-centric was a strike against Citadel Siege