Marvel counts among its characters such well-known superheroes as Spider-Man, Iron Man, Captain America, Wolverine, Thor, Hulk, Ant-Man, such teams as the Avengers, the Guardians of the Galaxy, the Fantastic Four, the Inhumans and the X-Men, and antagonists such as Doctor Doom, The Enchantress, Green Goblin, Ultron, Doctor Octopus, Thanos, Magneto and Loki. Most of Marvel's fictional characters operate in a single reality known as the Marvel Universe, with locations that mirror real-life cities. Characters such as Spider-Man, the Fantastic Four, the Avengers, Daredevil and Doctor Strange are based in New York City, whereas the X-Men have historically been based in Salem Center, New York and Hulk's stories often have been set in the American Southwest.

Captain America, Thor and Ultron all inhabit what fictional milieu?