The nation produced sufficient iron, but a primitive steel [iron with carbon added] was imported. It was scarce and expensive. Steel was used for tools, instruments, weapons and armor. Ships could carry about 300 people. Navigation was by simple charts that included wind direction for different seasons and the direction of north. The direction of the ship could be generally determined when the sky was clear by the position of the sun during the day or the north star during the night. Plays about miracles wrought by holy men or saints or the sufferings and fortitude of martyrs were performed, usually at the great church festivals. Most nobles could read, though writing was still a specialized craft. There were books on animals, plants, and stones. The lives of the saints as told in the book "The Golden Legend" were popular. The story of the early King Arthur was told in the book "The History of the Kings of England". The story at this time stressed Arthur as a hero and went as follows: Arthur became king at age 15. He had an inborn goodness and generosity as well as courage. He and his knights won battles against foreign settlers and neighboring clans. Once, he and his men surrounded a camp of foreigners until they gave up their gold and silver rather than starve. Arthur married Guenevere and established a court and retinue. Leaving Britain in the charge of his nephew Modred, he fought battles on the continent for land to give to his noblemen who did him service in his household and fought with him. When Arthur returned to Britain, he made battle with his nephew Modred who had crowned himself King. Arthur's knight Gawain, the son of his sister, and the enemy Modred were killed and Arthur was severely wounded. Arthur told his kinsman Constantine to rule Britain as king in his place. The intellectual world included art, secular literature, law, and medicine. There were about 90 physicians.