The U is a good place because you get them coming & going with better range.
My strategy was this:
4 waters at the very top upgraded to the 720 mark.
Then 6 waters 3 fire upgraded the same at the U.
ON the long straight a way at the top about 6 waters upgraded to 720.
Then around the corner before the bend about 5 waters upped to the one before 720.
That way you knock out a good chunk out the gate so by the time they make it to the castle theres only 1 left. So far my best is 78.
SO someone by any means post up different strategies cuz I’d like to get further.