Really simple, easy, and reasonable! Put an Archer on the point A in the following picture. After the Archer destroy two Air Defenses, deploy a Balloon.
In the following video, troops are upgraded. However, actually you don't need to do that. Goblins help you to clear this in short time.
First, put Archers on the pointed area in this picture. They attack and destroy 2 Air Defenses.
After Archers finish their job, deploy Balloons.
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[Level 17 - WatchTower] Hints & Tips