Good idea, but MANY things stand to be improved. A lot have already been mentioned (upgrading a whole line at a time, etc.) but I would also like to see the ability to go back to previous levels, a "save" button, etc. It's quite aggravating that you just keep going and going with no ultimate goal, then when you finally give up and lose there's no way to replay the level. It's just game over.
 "Torture Chamber III"){kind=icon}
 "Age of Defense"){kind=icon}
 "GRASS Real-Time Grower"){kind=icon}