Jelly Defense (iPhone iPod)
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Various Game Tips
Save the power ups for emergencies for when you're just about to lose your gems.
The blue-and-red units are incredibly useful during the early game - place as many as you can afford before moving onto single-coloured units.
You'll also need them for the 'My Favourite Flavor' (sic) Achievement, which requires you to complete a level using only one type of tower.