Galen has probably been the most enduring figure in the history of medicine, and yet he made many mistakes.
Born in Perganum in AD129, he was a very ambitious doctor, and used flashy showmanship to spread his ideas. He also wrote over 100 medical texts. These survived the fall of the Roman Empire because they were assimilated into Arabic culture, and eventually travelled back west into Europe via Avicenna's Canon of Medicine. Because his books didn't stress the polytheistic side of Roman culture, they did not offend the monotheistic Christians. Galen's books were accepted as religious doctrine by the Church, and arguing with them was a very dangerous thing to do.
Galen originally worked in an Asclepion and later a Gladiator school, where he got to see inside some very interesting wounds. He then increased his knowledge of anatomy by dissecting animals - at this point, human dissection was out of the question for religious reasons. This is why, despite his genius, Galen made mistakes. He described the rete mirabilis of an animal and thought it would also be found in humans, and described how the blood passed through the septum in the heart. This would later be disproved by Ibn al-Nafis.
Galen supported Hippocrates on the four humours and ethics, as well as the practice of clinical observation. He believed in treatment by opposites.