This is an excellent tower defense game. There are currently a total of thirty types of defense towers in the game, and you can choose 8 types of defense towers to carry in each battle. The energy will increase automatically with time and each tower can be upgraded up to LV3.
The game is challenging with increasing difficulty. You can upgrade your combat power from Teac Research and Landing Pod in Lab. The game currently has 7 main chapters and 2 sub-chapters, so you can enjoy the fun of tower defense game for a while.