To find the cache you must find the anwers to these questions, they can all be answered by standing in the middle of the square and looking around.
You need to count how many:
Towers = AB
Fountain walls = C
Statues = D
Lights designed by Chris Baldwin = EF
The cache can be located at:
N51 (C-F) (E+A) . (C+E) (D+B+D) (C+F) W002 (F+A) (D+F+B) . (B+B) (D+A) (C+D)