The Kremlin is a triangular, walled citadel in the center of Moscow
. The Kremlin is considered by most to be the heart of the city. First conceived in the 12th century, the Kremlin (which means fortress) was expanded by Tsar Ivan III (Ivan the Great) during the 15th century. His architects designed the magnificent Cathedral of the Assumption and the Faceted Palace, and the Kremlin was an interesting mix of both Russian and Renaissance styles. During the Soviet time of the 1930s, many of the Kremlin buildings were destroyed or vandalized, and the complex remained closed to the public until 1955.
Today the Kremlin is home to the Russian President and his administration. Many buildings are open to the public, but you may need to be with a guide (check in advance).
I visited the Kremlin when in Moscow on a wonderful Russian Waterways cruise tour from St. Petersburg.
The Kremlin was also one of the 21 finalists for the New Seven Wonders of the World.