"was conical" is not supposed to be parallel with "was banked".
"Banked with dirt to a height of three to four feet" is an attribute of the framework of poles
The parallel construction required here is "framework of poles overlaid with slabs... and banked with dirt..."

Then how do you choose b/w A & B?

Well that choice should not be difficult.

"as high as that of" is an awkward construction in this context.

Consider the following sentences.

1. My dog can jump to a height of three to four feet.

2. My dog can jump as high as that of three to four feet.

Which one do you think is correct from the two above?

Hope that helps.

A makes sense.
nice explanation by sethids tho..