Unter den Linden early 20th century, Wikipedia
History
The street has a long and storied history that dates back to the Middle Ages. In the 13th century, the street was called "stræte ût dem linden" (street of the linden trees) and was used as a market place for the surrounding village of Cölln. In the 17th century, the street was renamed "Unten der Linden" and became a fashionable promenade for the city's elite.
During the 18th and 19th centuries, Unten der Linden was known for its theaters, operas, and other cultural institutions, including the Royal Opera and the Royal Theater constructed under the reign of Fredrick the Great. The street also became a popular shopping destination, with many high-end stores and boutiques opening along its length, especially along Friedrichstrasse.
In the 20th century, the street underwent significant changes. After World War II, the street found itself in East Berlin and much of its Prussia and Imperial era history was either destroyed, left destroyed. The street was reunified in 1989, following the fall of the Berlin Wall. Today, Unten der Linden is once again a major cultural and commercial hub in Berlin, with many popular attractions, such as the Brandenburg Gate, the Berlin State Opera, the German History Museum, and Museum Island.
Personal Experience
A visit to Berlin is just incomplete without a stroll down Unter den Linden and lucky for you the street no longer has endless construction on it. The U5 subway line is complete and most major renovations are complete. I would suggest though going with a tour before heading out alone, but if you do want to hit the street on your own do it early or late. Unter den Linden - though far from the chaos of other major cities - can still get quite busy and some picturesque views are best spent with less noise.