The purest expression of tower defense gameplay is the free flash game Desktop Tower Defense.
The idea of laying out towers and watching them shoot at enemies, just so you can process this data and figure out even better ways to lay out towers next time, may not sound all that "fun." But if you are a person who likes machines, who's mind siezes on complex problems that need solving, who has a very "systemic" way of thnking . . . and I pause here to rephrase that as "But if you are Sam" . . . then these games represent less of a "recreation" and more of a "danger to your health and productivity."
Like Double-Stuff Mint Oreos, I often suspect that the Tower Defense genre was created by the Universe as a way to keep me, personally, in check. Like a well-placed frost tower, these games slow my progress so I can be more effectively dealt with.*
And I guess it's working, because between Savage Moon:
Tower Defense games seem to be about all I'm playing lately.
*I wonder if the designers who work on pedestrian malls and other public areas enjoy TD games. Seems like a similar skillset, using the environment to control a horde. I like to imagine one of these people looking a map of a shopping mall's food court and thinking "We're getting too many people bunched up at the entrance, I'll have to install some Cannon Towe . . . I mean some railings.