@dimohelloo0o0 - you can only have air unit upgrades on certain levels. I think its really dumb b/c if you've earned it you should be able to use it all the time. However, it would make the earlier levels increibly easy, especially the iron.
For Noobpwning247: For level tten try not to use so many tower soldiers. When they die they are made into skeletons which put more pressure on your Defense.
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