Let towers range be equal to archers range. Reducing the range of an archer garrisoning a sentry defense tower is incorrect. The elevation bonus is, i guess, the bonus range the unit has by garrisoning the tower and shooting arrows from the top of the defense tower, thus an unit equipped with bow will have bowmen range 72 mts + elevation bonus equal to a hypothetical tower height.
i hope this replay is clear enough to understand which the issue with sentry towers is:
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