How do I upgrade Towers?
If you are seeing a padlock when trying to upgrade your tower mid-game, you will need to spend Coins in the Upgrade section of the game to unlock further upgrades for your towers and allow them to become stronger. These become necessary as you progress through the game and face harder Creeps so pump those towers up!
You can also purchase Fusion Cells to give you an added boost to your level. These can be found in the “Shop” and used by tapping on the “Fusion Cell” icon in the bottom right side of the screen during a mission.