Besieging the City - "Opponent may not draw cards or make a rescue attempt next turn. "
Thaddeus - "Protect all cards in play, set-aside area, Artifact piles, hands, and decks from Evil Characters with toughness X or less. Cannot be interrupted."
Thaddeus protects cards whereas I believe Besieging the City targets your opponent as a player (so I think it would be classified as a restrict). As such, I don't believe Thaddeus would protect from it since he doesn't protect his controller directly.
*instaposted*
