Real estate tycoon Donald Trump participates in the Republican presidential primary debate on Thursday at the Quicken Loans Arena, in Cleveland. Getty Images

Real estate tycoon Donald Trump has long said he wants to build a wall along the border between the United States and Mexico to keep out immigrants. But during the first GOP primary debate Thursday night, he made a concession: That wall can have a "big, beautiful door" for people to come into the country legally.

His comments instantly went viral on Twitter, with dozens of users posting photos of famous doors like the one to the fictional world of Narnia. Trump was already in hot water over his previous comments on immigration. In his campaign kickoff speech, he referred to Mexican immigrants as rapists and criminals. Trump later said he was sure to win the Latino vote and that he employs "hundreds of Mexicans," so they love him.

He's repeatedly mentioned his plans for the wall, which he wants Mexico to fund. "I'm gonna say, 'Mexico, this is not going to continue; you're going to pay for that wall,' and they will pay for the wall," Trump told Bill O'Reilly this week. "And Bill, it's peanuts, what we're talking about."

On Thursday, all eyes were on Trump, who has quickly become famous for his controversial comments on topics like immigration and Sen. John McCain's time as a prisoner of war. A Fox News poll showed the real estate tycoon headed into Thursday's debate with the support of 26 percent of likely Republican primary voters, beating out former Florida Gov. Jeb Bush by 11 percentage points.

Trump told reporters he planned to be "very civil" during the debate. He recently made headlines for calling Sen. Lindsey Graham, R-S.C., a "stiff" and "lightweight," and criticizing Wisconsin Gov. Scott Walker's handling of his state's budget deficit. But politicians weren't Trump's only targets: He's insulted Russell Brand, Cher, Seth Meyers and Jerry Seinfeld, among others, USA Today reported.

Trump didn't go into Thursday's event on the offensive. "I don't want to attack anybody. Maybe I'll be attacked and maybe not," he told ABC. "I'd rather just discuss the issues."

Check out some of the best door-wall tweets below: