Hanging Gardens of Babylon, depicts the Tower of Babel in the background. By Marten van Heemskerck, 16th century

Detailed descriptions of the Gardens come from ancient Greek sources, including the writings of Strabo and Philo of Byzantium. Here are some excerpts from their accounts:

"The Garden is quadrangular, and each side is four plethra long. It consists of arched vaults which are located on checkered cube-like foundations.. The ascent of the uppermost terrace-roofs is made by a stairway..." "The Hanging Garden has plants cultivated above ground level, and the roots of the trees are embedded in an upper terrace rather than in the earth. The whole mass is supported on stone columns... Streams of water emerging from elevated sources flow down sloping channels... These waters irrigate the whole garden saturating the roots of plants and keeping the whole area moist. Hence the grass is permanently green and the leaves of trees grow firmly attached to supple branches... This is a work of art of royal luxury and its most striking feature is that the labor of cultivation is suspended above the heads of the spectators".

For a documentary on the Seven Wonders:

http://www.cosmolearning.com/documentaries/the-seven-wonders-of-the-ancient-world-417/

and about the Hanging Gardens:

http://www.cosmolearning.com/documentaries/hanging-gardens-of-babylon-460/