The triumphal arch at the western end of Unter den Linden, designed by Carl Gotthard Langhans and completed in 1791, is Berlin's most recognised monument. Napoleon rode through it in 1806 after defeating Prussia; his troops removed the Quadriga (the four-horse chariot on top) and took it to Paris. The Prussians retrieved it after Napoleon's defeat and added an iron cross to the goddess's staff.
The gate is at the heart of the new Berlin — the Holocaust Memorial is a few hundred metres south, the Reichstag is a short walk northwest, and the whole area is well-maintained and walkable. The Pariser Platz around the gate is quieter in the early morning before the tour groups arrive. Free to visit at any hour.