What did Leonardo da Vinci do for the king of France?

His help in engineering. In 1482, at the age of 30, Leonardo entered the service of the Duke of Milan. He lived and worked in Milan for 17 years, until Duke Ludovico Sforza's fall from power in 1499. It was during these years that he first explored his engineering talents by working as a military engineer. He studied the design of weapons, tanks, submarines, and other instruments of war. His architectural skills surfaced during this same period in his designs of domed churches, fortresses and canals. His eclectic inventor/engineer's mind was also focused on geometry, flying machines, mechanics, hydraulic, botany, diving, physics, and anatomy.