In "The Avengers," Earth's mightiest heroes had it easy. All they had to worry about was the single threat of Loki -- and, admittedly, his invading alien army. But one well-deployed nuclear bomb shot from New York and carried through to the far side of the universe took care of that problem right quick.

But in the upcoming sequel, "The Avengers: Age of Ultron," Captain America and friends will face deadlier threats -- and yes, that's "threats," plural, as in more than one. At least four, in fact, as "Age of Ultron" features incoming HYDRA menace Baron von Strucker (played by Thomas Kretschmann) and his cohorts, "The Twins," super-siblings Quicksilver (Aaron Taylor-Johnson) and Scarlet Witch (Elizabeth Olsen).

All three of those characters debuted in the "Captain America: The Winter Soldier" mid-credits sequence, and even though Quicksilver and Scarlet Witch are best-known for their heroics, it sounds as if they'll start the film on the wrong side of the law.

Then, of course, there's the titular robot menace himself: Ultron, played by "The Blacklist" actor James Spader. The maniacal machine is one of the most hotly-awaited villains in the superhero movie scene, and writer-director Joss Whedon is having a field day with the character.

"I'm having a blast with Ultron," he told Empire in a new interview. "He's not a creature of logic -- he's a robot who's genuinely disturbed. We're finding out what makes him menacing and at the same time endearing and funny and strange and unexpected, and everything a robot never is."

Four villains for the "Avengers" sequel, assuming there aren't others in the works, and not even counting any other android allies Ultron might recruit along the way. Is that three villains too many? Is Whedon simply adding these characters to put a smile on fans' faces? Absolutely not, according to the man himself.

"I fiercely dislike the idea of just throwing in more people for the sake of doing that," he stressed. "But last time I had all of Earth's Mightiest Heroes versus one British character actor, and I needed more conflict."

Not that Tom Hiddleston isn't enough to carry the weight against the Avengers, but it's comforting to know that the sequel will be an appropriately super-sized affair.

"The Avengers: Age of Ultron" assembles in theaters on May 1, 2015.