Tower of Oblivion

The Tower of Oblivion unlocks at level 24. There's loads of loot but you'll have to get past a lot of enemies to get it.

Use the Skip Battle feature in Tower of Oblivion to progress quickly, you can see the battle stats afterwards if you are curious.

If you get defeated, maybe try again, the result may change, but remember you'll lose a magic water drop for each loss, these are refreshed over time or you can buy with gems. Probably best however, to upgrade some more your heroes or try a different formation.