If you enjoy defense games, you might want to check out StormWinds 1.5. It's the newer version of the original StormWinds, featuring more weapons and a new "campaign" mode.

It's different from other defense games in that instead of building a map full of weapons that automatically fire at enemies, you can only build up to four, of which you can only control one at a time. That means you have to pick carefully and work on your aim (if you've ever played Worms you'll be right at home).

As defense games tend go, the rhythm is predictable. You continue to thwart oncoming enemy waves, which get continually more numerous and stronger. Just remember, if you do spend all day actually beating the entire campaign, pat yourself on the back, pour a drink, get a breather, and get some work done.