Ultra City is the city that Ultron built on top of the ruins of New York City after it killed the Avengers. There are powerful cannons along the coast to destroy anything that approaches. It is patrolled and maintained by Drone Walkers and Drone Flyers. It is commanded by Ultron in his Citadel. The buildings are on rails and can be moved at any time. While Ultron has been defeated [1] the Avengers have struggled to find a way to revert the city to it's former glory, and ever present reminder of the world's failure to stop the Superbot.
Most recently Franklin Richards has used his reality altering powers to fix most of the world. This means that Ultra City no longer exists and New York has returned in it's stead.[2]
