The holographic city was a holographic model of a steampunk-style city, allegedly created by Gregory Magnus.
It was discovered after Will Zimmerman encountered Gregory in the spirit world during the Big Bertha crisis, and was given clues that helped Helen Magnus to find the items necessary to bring the hologram to work.
Magnus and Nikola Tesla were later able to crack the description of the city, which turned out to be an advanced computer system. The system locked them out when it discovered source blood in their bodies and left behind a particle accelerator that used electromagnets to reach sufficient power that would destroy them and the Sanctuary, but they were able to circumvent it with an EM field created by Tesla. Once in, they found a holographic map of a hollow Earth by saying Gateway (kha harag).
It was given to the vampire queen in exchange for Nikola Tesla's life. When the queen tried to actavate it, the particle accelerator likely exploded and killed the last of the vampires.