I think the tower is in South America, for two reasons
A) Amanda Holiday walked to the tower her accent would suggest she is from the southern United States. If she walked she could basically go in two directions, north into Canada or south into central and south America.
B) Now this is something I heard the cryptarch say " civilization once stretched from the southern tip of this continent to the far north."
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I'm pretty sure you can see bannerfall from the tower