If the enchanted creature gains flying, Trapped in the Tower is put into your graveyard as a state-based action. The creature won't get stuck in the tower again when it loses flying.

Activated abilities contain a colon. They're generally written “[Cost]: [Effect].” Some keyword abilities are activated abilities and will have colons in their reminder text. Triggered abilities (starting with “when,” “whenever,” or “at”) are unaffected by Trapped in the Tower.