The Kremlin is the former palace of the tsars. The walls around the Kremlin (fortress) were first erected in the 10th century and were made of wood. In the 13th century brick walls were added. The modern Kremlin, as it is now, was built in the 16th century and it was the second most powerful castle in Europe after the one in Milan. Also, in the 16th century most of the decoration works were made, and the four famous cathedrals inside the Kremlin were built in this period. Nowadays it is where the president of Russia works.