In which country did the original stupa evolve into a pavilion-like watchtower?

I believe that this was China.

Stupas originated in India around 4th century BCE and later the idea spread around Asia . In China it was then combined with the Chinese Watchtowers, and then as such it spread (under the name Pagoda) to other countries, such as Japan.

Perspective is important because it enabled the 3 dimensional world to be translated into the 2d world of the picture plane. this had an immediate impact on the representation of reality in art and design. 

Did you read the book