If you've been out of the RTS loop for a while, you may not know that games like Defense Grid are called Tower Defense games. The Tower Defense "genre" has been especially popular here at DC. We even have
a list of TD games with reviews or short descriptions.
My personal favorites are:
Defense Grid: The Awakening.
Plants vs Zombies.
Master of Defense.
Onslaught.
Defense Grid and Plants vs Zombies are both available on Steam. Master of Defense and Onslaught are both found in the list I linked to above.
And a quirky fun one was created here for the DC Accessibility Game Contest:
Zombies Ate My Peoples. It's a switch game, which means you control the entire game by pressing just one button.