The Tower of Oblivion is a part of the game which is mostly PvE, with a leader board which grants no bonus rewards (The player can make progress, layer by layer, by defeating an enemy team at each layer.) The Tower of Oblivion unlocks at Level 24.
Current max layer is 500.
Reward on every layer : Gold + Promotion stones
Bonus rewards every 5 layers : Hero Shards or Gear.
5-Star Heroes as rewards are at Levels 105, and every 25 level after that.