Gate of Babylon

The Gate of Babylon, the King's Treasury, is one of the major Noble Phantasms of Gilgamesh. It is a key-shaped sword that connects to the "golden capital".

By connecting reality to the vault, he can pick and use the items in it as he pleases. Needless to say, the Noble Phantasm becomes more powerful with the wealth of the owner.

Similarly, there probably is a Road of Babylon.

Source: Fate/Side Material encyclopedia entry (p.57)

Gilgamesh mainly uses the Gate of Babylon to shoot the countless weapons in it like a rain of projectiles, but there also are various items in it, such as a mirror; Fate/Zero also shows us not only the flying machine Vimana, but even something as trivial as wine, which he calls "wine worthy of kings" and "drink of the gods" (given his extreme haughtiness, this probably is untrue and just his extravagant claims. It could be that ancient Mesopotamians were master wine makers ). Gilgamesh can also pick the items one by one for specific use, of course.

The weapons are the prototypes of the more recent Noble Phantasms, meaning that he owns the weapons and artifacts in his treasury when he is still alive and some of them would become legendary and mythical weapons/artifacts. After he died, his treasury is raided or perhaps is given away by his successor and then spread throughout the world. While Cú Chulainn has the Gáe Bolg that became famous, Gilgamesh possesses a weapon that later became the Gáe Bolg, but wasn't famous at the time Gilgamesh owned it.

The weapons that are seen in the Gate of Babylon are the prototypes of Caladbolg, Dainsleif, Durandal, Gáe Bolg, Gram, Harpe, Houtengeki, and Vajra. The Gate also contains an invisible sword, a sword of ice that freezes the space it cuts, a sickle that drains away prana and passes through flesh and armor without causing physical damage, a mirror shield capable of reflecting a blast of prana, a three-edged sword, and a hammer larger than a person.