Ability for reference:
"If an opponent's special ability targets your hand, he must discard a card from his territory (except a Lost Soul)."
The ability cannot insert itself between other abilities. Therefore, it completes after all Hero abilities are finished activating. At that moment (so after the fort was put in play in this case), it completes and forces the discard. It does not happen at the moment of the Look, since it cannot insert between all of the activating Hero abilities.
By the time it does complete, the fort is a legal target, and must be discarded if there is no other option.
