As you progress through the game you will gain access to more powerful versions of the base Towers. These can be built by tapping on an existing Tower to open up the Tower Menu, and selecting one of the available options along the top.
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As you progress through the game you will gain access to more powerful versions of the base Towers. These can be built by tapping on an existing Tower to open up the Tower Menu, and selecting one of the available options along the top.