More than one year into Donald Trump’s presidency, his promise to build a wall along the United States-Mexico border is a step closer to reality. These slabs of concrete and steel are prototypes for the new wall. They were recently erected in San Diego, California, near the border.

During his presidential campaign, Trump vowed to build a solid wall extending the full 1,900-mile length of the border, from the Pacific Ocean to the Gulf of Mexico. The president, who visited the area to inspect the prototypes in March, says the wall will help stop people from entering the U.S. illegally, as well as curb drug smuggling.  

But critics say a wall won’t end illegal immigration. They point out that most of the estimated 11 million people who are in the U.S. illegally are visitors who didn’t leave when they were supposed to.

Building the wall is expected to cost up to $25 billion. So far, funding has been caught up in the broader battle over immigration in Congress, where Democrats and Republicans remain far apart over how to fix the nation’s immigration system.