Drop Archers on point A in the following picture to take out an Air Defense. Cannons will attack Archers, so you can add more Archers.
After the Air Defense is gone, just deploy a Balloon. This stage is really simple and easy.
In the following video, Archer and Balloon are upgraded to level 4.
In the video, the player drops 16 Archers. While Archers are attacking the Air Defense, Cannon are attacking Archers.
While Archers are attacking the Air Defense, the player deploy 2 Balloons.
The Air Defense is gone, and Archers were defeated by Cannons. However, Cannons cannot attack Balloons.
This mission is accomplished!
[Level 17 - WatchTower] Hints & Tips