You don't need more than 2 room in the hotel to work perfectly making the possibility of buying room useless since clinet come at the same rate that they go away.
level 8 can be done with a massive AA
and AA maximized is simply amazing so keep your AA and make a line or two at the opposite of your base this will keep away the most of the aerian wave out of your tower
prepare your AA defense at least one wave before the first aerian wave
i'm on the 12 level and it's kind of easy to keep going
the europe challenge is impossible, my teammate is eliminated too quickly and i must defend myself again more than 500 soldiers on the north of the england and against 200 in france -_-
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