It is a fortress, but not one built in solitude, for this Superman — the Giants’ superman — did not come screeching down to the planet. Although, he at times looks faster than a speeding bullet, more powerful than a locomotive and able to leap tall defensive backs in a single bound.

This fortress, born of nature, honed by sweat and steeled by strain, belongs to Saquon Barkley and it is remarkable; a thing of beauty. It appears impervious and without flaw. Massive yet sculpted. Bulging yet lithe. It is the temple others worship, awe-inspiring but unattainable because there is only one Saquon Barkley and only one Saquon Body.

“Very proud of my body,” the second-year running back and full-time physical specimen told The Post during a rare break at Giants training camp. “Put a lot of hard work in. I surrounded myself with a great team. Your body is what you need to produce at a high level in this sport, that’s why I take it very serious.”

Indeed, Saquon Serious and Saquon Body are one and the same. His upper half is tank-like, intertwining sinew and muscle mass. His lower half is astounding, the nexus a pair of impossibly massive, trunk-like thighs that look as if someone on the assembly line at Human Technology misread the sizing instructions. The total package has teammates agog and onlookers aghast.

“He’s swole,” running back Rod Smith said.

“You don’t think he can get any more fit but he somehow did,” said Jon Halapio, a 6-foot-4, 315-pound center. “We only hope to look like that one day. He has the body to show off. I don’t know how it feels.”

Others could put in the work and never look like Barkley. So it goes in nature. There is body for the beach and body for the backfield. Barkley turns heads walking on the sand and waltzing through the secondary.

“There’s a lot of IG models and IG guys who work out and they look the part but could never do what any of us do on the football field,” Barkley said. “I know guys who don’t have six-packs but their core is ridiculously strong. For example, a lot of our linemen … they’re not ripped and shredded. You’re not really focusing on ripping it out or looking lean and looking crazy strong but you understand the work that you’re doing, you’re doing it to be a better football player and it happens to help your body look better.”

Barkley is ripped and shredded and crazy strong. At Penn State, he set a weight room record of 405 pounds in the power clean. He does not train like a power lifter but is confident he could squat 700 pounds and bench press close to 500 pounds.

It is his thighs that set him apart. No one ever called him “Say-quads” before Odell Beckham Jr. last year hit the rookie with it. Beckham is gone, traded to Cleveland, but his nickname handiwork remains, although Barkley gets it more from outsiders than he does teammates. When he did some offseason work with Baker Mayfield, the Browns quarterback taken No. 1 in the 2018 NFL Draft (Barkley was the second pick), Mayfield shortened it up to “Quads’’ when calling out to Barkley.

“People always said I have big legs or I would always get compliments on my legs but once I got here, now I get noticed by my legs,” Barkley said, smiling. “I remember I was walking through an airport and a woman complimented me, like ‘Oooo, you have nice legs.’ I was like … ‘Thank you?’ You don’t really know how to react to that.

“Or you see guys look at your legs. It kinda grew. I don’t care, it doesn’t bother me.”

The quads do not have minds of their own but they could be assigned separate zip codes.

“They’re ridiculous,” Smith said.

They are taut and protruding and measure nearly 29 inches around, absurdly immense for a person that wears size 36 pants. In shorts, Barkley looks as if he is powered by matching pistons.

“I have people reach out to me all the time going ‘I know you work with Saquon, how do I get quads like that?’” Ryan Flaherty, vice president of performance at Nike and Barkley’s trainer, told The Post. “I am like, ‘You didn’t get a chance to pick your mom and dad.’ A lot of that is genetic. I’ve never seen quads that size. He’ll come into a gym or a facility and coaches walk up to you and say, ‘In real life they are even bigger than what you see on TV.’ It’s pretty amazing to see those things in person.”

What is not so amazing is finding threads for those things. Fitting Barkley’s musculus quadriceps femoris with clothing that provide the style and feel he craves is, well, challenging.

“Especially when it comes to like high-end fashion, designer brands, it’s too narrow,” Barkley said. “So there are a lot things I can’t wear. I can’t wear some stuff Odell wears. For me, Amiri jeans is like something that just fits me perfectly. Or Keiser Clark, their jeans just fit me perfect. But if I had to put on like Balenciagas, sometimes those pants just don’t fit me well, unless it’s like the track pants and the loose-fitting ones, but it’s hard to find ones that fit slim and give me a good look.”

Custom-made clothing would soothe this search, but Barkley, other than few tailored suits, is having little of it.

“Nah, I’ll just shop around and find the right pants that fit me,” he said.

There are muscles for showing and muscles for going. Barkley has it all, which is why he gained 1,307 rushing yards, caught 91 passes for 721 yards and scored a combined 15 touchdowns as the NFL’s Offensive Rookie of the Year.

There is one word Barkley uses to explain why he crafted his quads into such weaponry.

“Power,” he said. “When I mean power I don’t mean running people over. Powerful being when you hit me, you feel the density and you can feel ‘em and it keeps me moving. Why I’m able to squat so much, why I’m able to do the things I do, why I’m able to make a jump-cut then make another jump-cut over somebody else. All those things comes with flexibility, balance, core stability, mobility in all the places you need to be, especially hips, to be able to bend and move that way, but the power comes from your quads and your hamstrings and your legs and that’s what you have to have as a running back.”

Barkley, standing 6-foot, wanted to challenge himself and get down to 224 pounds after playing as a rookie at 233. His body-fat index of slightly more than 4%, though, actually made losing weight too dangerous and he arrived this summer at 232.

“The combination of size, speed, quickness, explosiveness,” Flaherty said, “is about as rare as it gets when it comes to the human body.”

Yes, Saquon Barkley is indeed human. A different-looking human.