The Ultimate line of comics takes place on Earth-1610 within the Marvel Multiverse.
At first the Ultimate Universe was a simple updating of the Marvel Universe, changing radiation into genetics. Then they slowly revealed that all the Hulk, Spider-Man, Green Goblin, even the mutants, were created because of Captain America's creation. A Super Powers Arms race.
They included things like weaponizing the Pym Particles, creating Super Soldiers who had Giant Man's powers. And then there were several Iron type Men. But they couldn't recreate Captain America.
The End Of The Ultimate Universe
Due to the Secret Wars event in May 2015, Earth 1610 has merged with Earth-616 to create an All New Marvel Universe.