What are the established rules set for a person to Become a Buddha other than, that, he should have lived as a King?

Long time ago , I was reading some text related to Buddha online and saw that there were many pre-conditions already set for a person to become a complete Buddha. I remember very clearly , the first condition was that the person should have lived as King in the past. While googling again to see the other conditions, I am unable to find the original article. Is anyone aware of such conditions ? Can someone help . Thanks