Question

Used as a warning system in the old north church (describe it in one word)

Answer 1

Built in 1723 and oldest standing church in Boston.

Answer 2

In the Old North Church of Boston one lantern would be lit if the British were coming by land and two of them lit if they were to come by sea.
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One word I would use to describe this is "tactical" or "strategic."
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Didn't really understand what you were asking, but I hope this helped somewhat.