Keep in mind that a Tower Duel can end prematurely, despite any of the players having attempts available. This is called Checkmate and occurs when one player has gained such an advantage that the other player cannot catch up, even if they use all of their available attempts.
The outcome of Tower Duel by Checkmate will depend on the number of attempts you have and the amount of categories you have won.
For more information about this game mode, we recommend the article How can I play the Tower Duel mode?