Short answers, pulled from the story.
The Brandenburg Gate was built between 1788 and 1791 by order of King Frederick William II of Prussia. The architect was Carl Gotthard Langhans, who based the design on the Propylaea, the gateway to the Acropolis of Athens. The bronze quadriga on top was created by sculptor Johann Gottfried Schadow.
Napoleon took the quadriga to Paris after the Prussian defeat at the Battle of Jena-Auerstedt in 1806. It was returned to Berlin in 1814 following Napoleon's defeat, when General Ernst von Pfuel led the Prussian occupation of Paris. Karl Friedrich Schinkel then redesigned the quadriga to reflect the gate's new identity as a Prussian triumphal arch.
Thousands of people gathered at the wall to celebrate its fall on the 9th of November 1989. On the 22nd of December 1989, West German Chancellor Helmut Kohl walked through the gate to be greeted by East German Prime Minister Hans Modrow. That same evening, conductor Leonard Bernstein led the Berlin Philharmonic in a performance of Beethoven's Ninth Symphony at the gate, replacing the word Freude with Freiheit in the final movement.
On the 12th of June 1987, U.S. President Ronald Reagan spoke to the West Berlin populace at the Brandenburg Gate and directly addressed Soviet leader Mikhail Gorbachev, saying: "Mr. Gorbachev, tear down this wall." Reagan called on Gorbachev to raze the Berlin Wall as a gesture toward peace, prosperity, and liberalization.
The Brandenburg Gate is one of the earliest examples of Greek Revival architecture in Germany, modeled on the Propylaea in Athens and supported by twelve fluted Doric columns. The quadriga on top originally represented Eirene, the Greek goddess of peace, but after the Napoleonic Wars was redesignated as Victoria, the Roman goddess of victory, and equipped with an Iron Cross standard and a crowned Imperial eagle.
The Berlin Wall, constructed beginning on Barbed Wire Sunday, the 13th of August 1961, passed directly by the gate's western side and kept it closed throughout the entire Berlin Wall period. The wall around the area was demolished after the border crossing was reopened on the 22nd of December 1989, and the Iron Cross was restored to the quadriga after German reunification in 1990.