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.

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.