White text shows your money. To select what to build, click at the floating icons at the top of the screen(buildings price is shown by white text under the icons). To build - select the building that you want to build, and click at the place where you want to build it(when you build a building, you couldn't move a half of a second, and you have a restricted building range).
First building is a wall, second is a turret.
Don't bunker, build mazes.
Right-click to destroy your building.
Hold your finger at screen. The cube will move at the point you are tapping. Double click to build, click to select what do you need to build(at the icon)