Here is the difference between the description of the guard tower upgrade and the keep upgrade -
Upgrade to Guard Tower:
Upgrades your Watch Towers and lets you build Guard Towers, which are stronger and have more attack strength.
Upgrade to Keep:
Upgrades Guard Towers and lets you build Keeps, which are stronger and have more attack strength, range, and armor.
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All that being said, there is no current range increase as described in the description.
Whether it be +1 or more is besides the point, it’s simply missing.