The Department of Homeland Security this week issued requests for proposals for President Trump's planned wall along the U.S.-Mexico border, inviting groups to pitch their designs, prototypes and plans for constructing such a wall.

According to Reuters, the request for proposals demands that the wall be 30 feet high, built with concrete and strike a duality between being both "physically imposing" and "aesthetically pleasing in color."

The primary request is for a 30-foot wall, but U.S. Customs and Border Protection posted a document saying it would accept designs at 18 feet tall.

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"Designs with heights of less than 18 feet are not acceptable," the document said, according to Reuters, adding that the wall should have features that do not allow people to climb over it and should prevent digging below the wall.

"The wall shall prevent/deter for a minimum of 1 hour the creation of a physical breach of the wall (e.g., punching through the wall) larger than 12 inches (30 cm) in diameter or square using sledgehammer, car jack, pickaxe, chisel, battery-operated impact tools, battery-operated cutting tools, oxy/acetylene torch or other similar hand-held tools," the document read.

The wall was a major campaign promise from Trump, who had vowed that Mexico would pay for the wall, something that the country has rejected.

The Hill reported this week that GOP leaders in Congress want more details from Trump about his proposed wall before appropriating significant funding for the project.