I was messing around and having played England with all castles I decided to have a joke attempt at world domination with Milan with only cities, and as has been said above there militia are really good. By being really very aggresive and wipin gout the French royal family by turn 20, (two assasinations and one battle near Genoa) I was really big really quick, with most of Italy (bar Rome) and France (Bar Cean, Rennes and Paris) taking the Iberian penninsular was easy after that I could swarm people with my militia. This was on M/M but for me the biggest advantage isn't the militia but your position, I really like assasins and because diplomats etc. tend to gather in Northern Italy it is really easy to get near full assasins quite quickly, this means you can have 4 top level assasins by turn 25 and with them taking out factions the easy way is well easy.