Really this deserves 1.5 stars, but that's not an option.
Where to begin... the gameplay itself is serviceable enough, with no real downsides other than its extreme simplicity. Everything functions properly, and it's really neither easy nor hard. But then there's other issues...
The biggest one is with the ads. I understand that free games need the ads to make profit, that's fine. But rather than put the ads at the beginning and/or end of any given run, the ad show up in the middle of gameplay, between waves. That's the worst possible place for them to show up other than in the middle of a wave itself!
Then, there's the AI. Turrets seem to lock onto targets the moment they come into range, and refuse to fire on anything else until their target leaves their range. This is fine with most damaging turrets, but extremely frustrating with the slowdown turrets as you watch one target standing still as others zip by unslowed.
The most despicable of all though is that this game exists twice on the App Store, under different names, probably as an attempt to bypass the one-star rating the other copy got. Just search "Tower Defense Games", you'll find it easily enough.
Add in turrets that fire backwards, no way to save, and typos in the description, and you have this game.