I think it must have been all the harbors I built that helped my economy. I also really packed in the cities on the starting continent, to keep other civs off of it. As a result, most of those cities used a lot of water squares, generating trade. I built on lakes to avoid building aqueducts where I could, but that just helped in a few cities.
Since I didn't make much military through the middle ages, a lot of production went to harbors, marketplaces, libs, etc. I think every city except a couple inland/freshwater ones had a harbor. I just had one Medieval Infantry war to take two cities the dutch dropped on my continent (I mostly using warriors, upgraded to med. infantry). I had originally produced those warriors for fending off barbarians and for happiness.
I had NO military for a while. I guess in a harder level I probably couldn't have pulled that off. It helped being on an island. There was one turn in early M.A. where just about everyone demanded tribute. I refused all and no one declared war, and the next turn I upgraded all my warriors so i had an "average" military again.