The Kremlin is Europe's biggest functioning fortification, with five palaces and four cathedrals. 6. In 1990, the Kremlin and Red Square were designated as UNESCO World Heritage Sites. Except for two, the Kremlin's 20 towers all have names. They include the Ivanov Tower, named after Ivan the Great (r. 957-963); the Luzhkov Tower, named after Yury Luzhkov (b. 1945); the Malyshev Tower, named after Alexander Nevsky (1238-1314); the Mizin Tower, named after Mikhail Mizin (d. 1430); the Obolon City Palace, which serves as the headquarters of the Moscow city government. The Kremlin consists of over 40 buildings, including halls, churches, and museums. It covers about 140 acres (0.5 km2) on three hills: Red Square is at the heart of the complex; to its south is the Upper Town, which includes the State Kremlin Palace and the Cathedral of St. Basil the Blessed; and to the west is the Lower Town, which includes the Armory Chamber and the Church of St. Nicholas Outside the Walls. The word "kremlin" comes from Russian kreml', meaning "citadel". Before the construction of the Kremlin, Russians defended themselves from invaders by building fortified camps known as pidaras.