Retro Defense Lite for iPhone

From Larva Labs:

Retro Defense is hours of action and strategy for your iPhone or iPod touch.However, if you're new to this type of game, your goal is to stop the invaders from getting through the board. You know the path they'll take, so you have to spend your money wisely and build the right towers. Each tower type has it's own benefits and weaknesses, and there are bonus towers and freeze towers to slow down the invaders and help you destroy them. Tower placement is key, as well as planning properly to not get caught by the wrong color of invader.

Note: The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries."