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.
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.