Crossword Genius

Not the architectural style associated with Pisa's tower? (13)

Ross

I believe the answer is:

perpendicular

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'tower?' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?

I don't understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for perpendicular that I've seen before include "Sort of architecture" , "style of construction" , "At ninety degrees" , "Gothic style following 7" , "Vertical - at 90 degrees" .)

I've seen this clue in The Telegraph.
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