On "This Week," Stephanopoulos pointed out that the former Obama HUD secretary said in a debate last week that he would decriminalize the act of crossing the border illegally.
"I know you reject the rhetoric on open borders, but isn't that effectively open borders, not limiting immigration in any real way?" he asked.
Castro responded by saying his plan would not end border enforcement or the civil court process that leads to deportations.
"There's no way that we can call that open borders because we have 654 miles of fencing, we have thousands of personnel at the border, we have planes, we have helicopters, boats, security, cameras, guns," Castro said.
"That's by no stretch of the imagination, open borders. And then, you know, secondly, there is still a civil court process. There are still people who are being deported. There are people applying for asylum that do not receive or not granted asylum."
Castro said the idea that he supports "open borders" is a "right-wing talking point and always has been," arguing that President Trump and Republicans will make the accusation no matter what Democrats propose on immigration.
Castro generated some buzz at the first debate last week by forcefully calling out former Rep. Beto O'Rourke for failing to support the decriminalization of border crossings.
After the debate, Google saw a 2,400 percent spike on Castro searches. Castro said Thursday he believes his campaign saw its best fundraising day of the campaign after the debate.