(CNN) Former Housing and Urban Development Secretary Julián Castro had some direct advice for President Donald Trump when asked on Thursday: "Follow the law."

Castro's CNN town hall in Washington, DC, revolved around Trump, from immigration to climate change to the swirl of legal questions surrounding the President.

That is why, when asked what "one idea or piece of advice" he would give Trump, Castro said, "Follow the law. For any other president that might sound like a joke, right, but look what this President has tried to do."

The exchange made clear for Castro, who has so far failed to break out of the large pack of Democrats running for president, sees taking on Trump as the best way to seize his moment on national TV.

Focus on Trump

Right off the top of the town hall, Castro responded to a new report that the White House pressured ICE to move undocumented immigrants to sanctuary cities or top Democrats' districts.

"The cruelty of this administration never seems to end," he said.

He added, "Now they're talking about busing families to particular cities to target political opponents. You know, these folks want us to choose cruelty as a weapon against these people and against political opponents and last week I released an immigration plan. I'm calling on Americans to choose compassion, not cruelty."

Castro's focus on Trump also included the President's tax returns, which Democrats on Capitol Hill have recently tried to get their hands on.

Castro said Thursday that he believes presidential candidates should be required by law to release their taxes in order to ascend to the presidency.

"I support making a requirement by statute. Congress passing a law that requires people who are running for president to submit 10 years of their tax returns," Castro said. "It is astonishing that this President still has not released his taxes, even though he said, at one point, that he would."

Castro went on to say that he hoped congressional Democrats are successful in getting the President's tax returns and that he would release 10 years of his tax returns in the next few weeks.

Castro also rebuffed Trump on immigration, arguing that the country can "have a secure border" and "maintain that security," while also being compassionate.

Trump's immigration message became an anchor of his 2016 campaign and, since winning, the President has continually upped his immigration rhetoric and demanded that the federal government fund a wall along the US-Mexico border.

"This President wants us to believe that we have to choose between border security," Castro said. "I believe that our border is more secure than it's ever been. ... We can have a secure border and we can maintain that security. We can be compassionate as well."

'I won't assume you're a stoner'

Castro, during a colorful answer about marijuana, said that he would expunge criminal records for anyone imprisoned because of marijuana.

Castro supports the legalization of marijuana, but taking the step to clear the records of those charged under current drug laws is further than some Democratic contenders are willing to go.

"We need to go back and expunge the records of people who were imprisoned because of using marijuana. And this is important," he said. "This part is important, in part, because there are a lot of people, and folks in this audience probably know some of them who have served jail time, right, and disproportionately it's impacted communities of color and poor neighborhoods of people who have been imprisoned because of marijuana use. So it's not enough just to say we want to legalize it. We actually want to go back and expunge these records."

After CNN's Don Lemon noted that the question on marijuana came from someone whose last name was Stone, Castro laughed and said, "It's all right, man, look, your last name is Stone; I won't assume you're a stoner. My last name is Castro; don't assume I'm a dictator."

'The original sin of slavery'

Castro succinctly and directly explained his support for paying reparations to African-American families whose ancestors were slaves, telling the audience that payments are needed because the United States has "never fully addressed in this country the original sin of slavery."

"Because of that we have never truly healed as a country," Castro said. "If we compensate people under our Constitution, if we take their property, why wouldn't you compensate people who actually were considered property and sanctioned as property by the state?"

Castro went on to say that he supports legislation from Rep. Sheila Jackson Lee that would appoint a commission to determine how reparations would be paid.

Inspiring young Latinos

Castro reflected on being the only Latino candidate running for president in 2020 on Thursday, telling Lemon that he hopes his candidacy will inspire young Latino boys and girls to believe they could run for president someday too.

Castro, should he win the presidency, would be the nation's first Latino president, a fact he has said he does not take lightly.

Castro -- whose grandmother, Victoria Castro, was born in the Mexican border state of Coahuila and crossed into the United States at Eagle Pass, Texas, in 1922 after her parents died during the Mexican Revolution -- went on to say that he could "imagine the tears in her eyes" as she watches his 2020 run.

"I'm very proud of my background. And I think voters are going to make it the decision on a lot of things, your experience, your message, how everybody gets out their future and delivers their vision of the country, your track record," he said. "But to me it is meaningful to be able to run right now when the Latino community feels like this President has put a target on their back."

He added: "And my hope, is that not only will I be offering a different vision for the future of this... but also, that I could be some little Latino boys or girls that say, 'hey... if he's doing that I can do that, then I can do that too.'"

Castro has somewhat resisted being tagged at the Latino candidate in the race and his campaign has highlighted the fact that he is organizing all Americans, not just Latinos. But the campaign has also worked to organize Latino communities across the country, including in Iowa, where Castro has focused time on organizing the small communities of Latinos in the state's northwest corner.