Robocalypse TD

Other tips, not in the tutorial:  You can upgrade turrets by left-clicking on a turret you have already built.  The cost to upgrade a turret is located in the bottom right corner of the screen when your mouse hovers over that turret.  

Right-clicking allows you to sell turrets for 80% of the original price.

If in full-screen, you can pause with the 'P' button.