AD 122 - “Emperor Hadrian decided … to keep the Picts out … by building the Wall… This grand idea of a wall would have matched Hadrian's character and his desire to stamp his personality on the Empire. He had already decided to adopt radically different policies from his predecessors … From his reign the Empire effectively stopped expanding and the emphasis moved on to the defensive. The idea of a colossal fixed boundary in the far north of his Empire would have fitted ideally with Hadrian's new policy.” – www.explore-hadrians-wall.com

August 17, 1961 - The German Democratic Republic begins construction of the "Anti-Fascist Protective Wall" (Antifaschistischer Schutzwall). It would become known as the Berlin Wall. Rationalized to keep fascists out, its real intent was to keep East Berliners in.

"June 16, 2015 - "I will build a great wall -- and nobody builds walls better than me, believe me --and I'll build them very inexpensively. I will build a great, great wall on our southern border, and I will make Mexico pay for that wall. Mark my words." - Donald Trump

January 6, 2017 - "Multiple media reported on congressional talks over how to pay for the wall, including a CNN report Thursday night that said Trump's team wants to ask Congress for money to construct the wall ... Trump himself has estimated his border wall would cost $8 billion, though other analysts have estimated the price would be as much as $10 billion..." - LA Times

Now - For an interactive cost estimator of Trump's Wall, see:
https://www.technologyreview.com/s/602494/bad-math-props-up-trumps-border-wall/