Rules FAQ
- If the card cannot be put into play for any reason (because of its cardtype, because there is a copy of it in the dead pile, Barring the Gates, etc.), it will remain on top of your deck.
Citadel.
Reaction: After you win a challenge in which you control a participating Maester character, reveal the top card of your deck and put it into play.
Link: Decklists
Rules FAQ