You all heard of Vatican City, right there in the center of Rome. There are plenty of walls surrounding the city for protection against invaders. 

“It isn’t all surrounded by walls, and it’s not like you need a separate visa or a passport to enter,” he said. “You wouldn’t know, almost, when you even entered Vatican City. There is a white line painted on the ground in St. Peter’s Square, but that kind of thing is not obvious everywhere.”

There are, to be sure, formidable walls in Vatican City, and much of of the site, including the gardens and the modest guesthouse that is home to Francis, is set behind them. But the walls do not entirely enclose the city-state, and in the modern era they are not meant to, historians said.

“Anybody can walk into St. Peter’s Square — that’s the whole point of it,” said Dr. Mannion. “It was designed to be welcoming and to draw people in like two open arms, to draw them into the heart of the church.”

Some of the walls in Vatican City were built in the ninth century by Pope Leo IV in an attempt to protect it from attacks by pirates and other marauders, historians said. But other stretches of wall were built during the 15th and 16th centuries, Dr. Mannion said, less as a defensive measure and more as “a political and cultural statement” about the cultural and political power of the pope.

As highlighted above, there are some walls that were built to protect the Vatican from attacks by pirates and marauders; sounds like the wall proposed by Trump will do the same thing. And in ten decades there may be no need for the wall.