Classical architecture was formal, and it always obeyed laws. It used symmetry, which really means balance, and it used proportion between shapes. The Golden Mean was a rule which said, (to put it simply) if you are making a room, or any other thing, it will work best if you always make the long side 1.6 times as long as the short side. There are many 'laws' in classical architecture, like how high the middle of an arched bridge needs to be (which depends on how wide the bridge needs to be). These laws were learned from thousands of years of experience and they are often used today. However, today more notice is taken of specific facts, such as what wind speeds occur once or twice in a century. Several bridges have blown down because that was not properly taken into consideration.