Hmmm Defense Grid is a maybe right now. I do like tower defense games and it seems like it would be a good time-killing casual game.
This isn't just for you but I think it needs to be stressed:
IF YOU DONT HAVE THIS GAME AND YOU REMOTELY ARE INTERESTED IN TOWER DEFENSE.... GET THIS GAME!!!!!!!!!!!!!!!
Seriously, I keep coming back to it over and over. It is a huge bargain, it looks great, and you will put A LOT OF HOURS into it. Addicting tower defense gameplay done really well -AND- it's made with an engine that isn't flash (dev's: stop making cartoon tower defense games!).
There is so much value to be had in this title that it's silly not to have it.
And I also want to stress this: IF YOU BUY THIS GAME, DO YOURSELF A FAVOR AND PLAY IT! I am just as bad as the worst in here for buying something and letting it sit greyed out in steam. You can't do that to yourself with this game. Too much joy to be had.