The Castle on a Cloud

Creatures without flying can't attack you unless
their controller pays {2} for each creature that's
attacking you.
{2}{w/u}{w/u}: Until end of turn, The Castle on a
Cloud becomes a 4/4 white and blue legendary
artifact creature with flying, haste, hexproof, and
vigilance. Activate only if both {w} and {u} were
spent to cast this spell.