I have finally finished Kingdom Rush. I don't mean just beat it by getting through the campaign. I finished the campaign, finished all Heroic and Iron Challenges, extra content that was added later on, and I got every achievement in the game.
One thing is for sure, if I had time, I would always play this game. This is the best castle defense game on the iPad I've played and I have been playing a few. So don't pass up on getting this game.