Answer: It´s a reference to the Hanging Gardens of Babylon.
Step-by-step explanation:
The Hanging Gardens was an outstanding terrace of trees and plants in Babylon city. Along with the Tower of Babel and the Ishtar Gate, these garden seemingly floating over the city is one of the improvements the king Nebuchadnezzar made to the city, in an attempt to add beauty to a place already deemed as the Gate of God.