The Spaniard separated herself from the rest of the field early in the day with a first-run score of 87.75 and never looked in danger of relinquishing top spot in Copper Mountain.
Jiayu Liu was Castellet's closest rival, with her score of 80.000 securing the silver medal, while America's Massie Mastro completed the podium by taking third with 77.50.
And after claiming first place in sensational style, Castellet admitted she was delighted with the way she handled the weather to impress on the built 22-foot superpipe.
"Today was hard, so I believe my experience paid off well," Castellet said. "We knew the weather was coming in and there was already a bit of snow in the halfpipe when we started.
"So I decided to go with a strong run from the beginning instead of just doing a safe run to start out, and that paid off well. The weather really came in hard after that and my run held up, so I'm stoked."
In the men's event, Scott James picked up right where he left off last season as he claimed his third straight win in Copper Mountain ahead of Japan's duo Yuto Totsuka and Ruka Hirano.
The 25-year-old enjoyed a perfect six-for-six season in 2018/19 and delivered another standout performance to post a score of 96.00 – beating Totsuka by a margin of 1.50.
"It feels absolutely amazing," said James. "I had big expectations after last season, so coming in here I had to iron those emotions out and come out with a clear head, and approach it as a new season.
"So I did that and came out here knowing what I had to do to be on top. Obviously the elements weren't in our favour, but everyone rode really well like I knew they were going to."