My answer is as per follow: The building that are STILL at the first layer attracted the wall breaker.
If those buildings are cleared, it wouldn't happen.
Then again, if those buildings at the first layer that wasn't fully enclose is able to attract the wall breaker, then something is wrong.
But personally, i will destroy all the buildings first before moving on, so not an issue to me.
Skip to 1:10 for the actual battle.
*Ignore the 4th wall breaker, mis-judged the distance from that edge to the 3rd layer.*



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