I started playing a bit of Kingdom Rush two weeks ago following the news of the Flash game's iPad release, and managed to drag myself away from Ironhide Game Studio's excellent little tower defense title before an entire day was lost. Alas, resistance was futile as I was reminded of its existence by Rock, Paper, Shotgun this weekend.
Kingdom Rush takes traditional tower defense, puts it in a medieval fantasy jacket, and adds cute little guys you can spawn to obstruct the enemy waves à la Ninjatown on the DS. There's also a tech tree of sorts to keep you playing, upgrading your defenses and spells, and finally laughing maniacally as you go mad with power. You can play the Flash version for free over at Armor Games or nab the iPad version for $2.99.