Obsolete military units can be upgraded at half price.
May trigger Golden Age for Militaristic civilizations.
Leonardo da Vinci was one of the most celebrated painters of the Italian Renaissance. His most famous works include the "Mona Lisa" and "The Last Supper". Although best known for his art, da Vinci was also an accomplished observer and designer. Preserved in his sketchbooks are ideas for dozens of inventions, many of them centuries ahead of their time. These include designs for such devices as drawbridges, construction equipment, parachutes, and even helicopters. His scientific and technical drawings became the basis for the development of many of these modern devices.