Want to lose many, many hours to a tower defense game that'll hold you in helpless thrall? Check out either of the GemCraft games at Armor Games. They've got excellent core mechanics, appealing graphics and audio for Flash efforts, and RPG-lite skill systems that have caused a fair amount of sleep disruption for me over the past couple weeks.
Be warned: expect to invest a pretty ludicrous amount of time if you want to get to the very end of the more recent "GemCraft chapter 0" prequel.