Tower Defense Infinite Mode unlocks at Base Lv.10. You can enter this gameplay through the Battle Hall.
After entering the challenge, enemies will continue to attack from all different directions. You need to build different Defense Towers to fend off enemy attacks.
Your Buildings, Tech, Ultra Weapons and Equipment will all provide some additional damage. Appointed Generals will also increase the Defense Towers' damage. Once you entered the battle, you can no longer change Appointed Generals.