In my opinion it sounds good to let the plane fly left/right in turn. This has several advantages.
I find the game a little hard right in the beginning. Giving the player another chance of aiming is a good choice, plus, targeting the most left building is exactly as hard as targeting the most right one. I found myself always loosing because I had some buildings left on the left side.
You can let it get harder just by increasing planes speed in the levels, so the double hit chance would be balanced.
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