It is believed that the garden was built by King Nebuchadnezzar, who ruled the city for 43 years starting in 605 BC (there is a less reliable, alternative story that the gardens were built by the Assyrian Queen Semiramis during her five year reign starting in 810 BC). Nebuchadnezzar's reign was the height of the city's power and influence, and the King constructed an enormous array of temples, streets, palaces and walls.

The story is that the gardens were built to cheer up Nebuchadnezzar's homesick wife, Amyitis. Amyitis, daughter of the king of the Medes, was married to Nebuchadnezzar to create an alliance between the nations. The land she came from was green, rugged and mountainous, and she found the flat, sun-baked terrain of Mesopotamia depressing. The king decided to recreate her homeland by building an artificial mountain with rooftop gardens.

The Hanging Gardens probably did not really "hang" in the sense of being suspended from cables or ropes. The name comes from an inexact translation of the Greek word kremastos or the Latin word pensilis, which mean not just "hanging" but "overhanging" as in the case of a terrace or balcony.

The Greek geographer Strabo, who described the gardens in first century BC, wrote, "It consists of vaulted terraces raised one above another, and resting upon cube-shaped pillars. These are hollow and filled with earth to allow trees of the largest size to be planted. The pillars, the vaults, and terraces are constructed of baked brick and asphalt. The ascent to the highest story is by stairs, and at their side are water engines, by means of which persons, appointed expressly for the purpose, are continually employed in raising water from the Euphrates into the garden."

The most amazing part of the garden was the irrigation system: Babylon rarely received rain, and for the garden to survive it would have had to been irrigated by using water from the nearby Euphrates River. That meant lifting the water far into the air so that it could flow down through the terraces, watering the plants at each level. This was probably done by means of a chain pump - two large wheels, one above the other, connected by a chain. On the chain are hung buckets. Below the bottom wheel is a pool with the water source. As the wheel is turned, the buckets dip into the pool and pick up water. The chain then lifts them to the upper wheel, where the buckets are tipped and dumped into an upper pool. The chain then carries the empty ones back down to be refilled.

The pool at the top of the gardens could then be released by gates into channels which acted as artificial streams to water the gardens. The pump wheel below was attached to a shaft and a handle. By turning the handle slaves provided the power to run the contraption.

Construction of the garden wasn't only complicated by getting the water up to the top, but also by having to avoid having the liquid ruin the foundation once it was released. Since stone was difficult to get on the Mesopotamian plain, most of the architecture in Babel utilized brick. The bricks were composed of clay mixed with chopped straw and baked in the sun. The bricks were then joined with bitumen, a slimy substance, which acted as a mortar. These bricks quickly dissolved when soaked with water. For most buildings in Babel this wasn't a problem because rain was so rare. However, the gardens were continually exposed to irrigation and the foundation had to be protected.

Diodorus Siculus, a Greek historian, stated that the platforms on which the garden stood consisted of huge slabs of stone (otherwise unheard of in Babel), covered with layers of reed, asphalt and tiles. Over this was put "a covering with sheets of lead, that the wet which drenched through the earth might not rot the foundation. Upon all these was laid earth of a convenient depth, sufficient for the growth of the greatest trees. When the soil was laid even and smooth, it was planted with all sorts of trees, which both for greatness and beauty might delight the spectators."