Actually, that's been a vital part of B5's design from the very start.
The dead center of B5 is The Garden, similar to an O'Neill station, with food
and other crops grown on the 360-degree walls. It runs through the center of
B5, so the very VERY center is zero-G. It's through this center that the
shuttle runs.

A small joke in the script finds Delenn (the Minbari ambassador) in the
Garden, and someone asks him about it. He'd thought that Sinclair had named
it after just what you suggest, the Hanging Gardens of Babylon, which he's
read about...but it seems Sinclair named it after some other garden...a Square
Garden of Madison, or something like that...and he's trying to find the
cultural references and figure it out...probably thinks it's something to do
with mythology....

jms