Build a couple of armies. Just pillage.. Wherever you want. (I usually conquer Carthagians, then Barbarians (Germania, Dacia, Gaul and Britannia), then Spain, then Numidia..)

Just remember.. Have more armies and be on the offensive. You will have so much money that you will just bribe any attacking forces.. Remember to exterminate and try to keep taxes as high as possible, and always bribe/gift when above 50k