Castellanos tossed his helmet and was mobbed by teammates on the infield as a game that seemed to slip away one inning earlier turned into one more comeback for the NL East champions.
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Megill retired the first two batters of the ninth and walked Harper, who also homered and scored twice. Castellanos, who also homered, followed with perhaps the hit of his Phillies' career and sent the towel-waving crowd at Citizens Bank Park into a frenzy.
"Unbelievable. Unbelievable," Castellanos said. "If he blows a fastball by me so be it. I'd rather that than swing at something in the dirt. It was incredible but the series is even. Now we go to New York and there's a lot of baseball left."
Game 3 is Tuesday in New York, the Mets' first home game since Sept. 22.
Fan Reactions
After a tough loss on Saturday, Phillies fans were ecstatic when the team got a huge win against the Mets.
Citizen's Bank Park erupted when the game was over, many cheering and wildly celebrating.
"Let's go baby! Let's go!" members of the crowd cheered.
Fans told Action News the energy was electric.
"That was best game ever. I've only been to three, this was amazing for me," one woman said.
"Oh my gosh, it was awesome! Best game of my life," added Kevin Hutt. "Energy was like a rollercoaster, but ultimately awesome. Go Phils!"
October 6 seems to be an important date for the Phillies, as it marks their win against the Mets Sunday and also 14 years since the historic no-hitter game.
Either way, fans are ready for what's next.
"Bring on game three!" one fan cheered. "Phils in four, Phils in four!"
The Associated Press contributed to this report.