Papyrus and beer are your main exports, but you can also import flax, build Weavers to turn it into linen, and export the linen for a healthy profit. Also watch your food levels. You might consider using some of your profits to buy extra food as your population grows. If you decide to import food, remember to tell the city's Storage Yards to accept the particular food you are importing. You'll have to build a navy to defend your city from attack.
Warships attack waterborne assailants.
To build a warship, you must first build a Warship Wharf. Then, make sure the Shipwright has a supply of wood. With a supply of wood, the Shipwright will begin building the warship as soon as the Warship Wharf is placed. It's also a good idea to have soldiers at the ready in case enemy transport ships make it past your warships and land their troops. You can build on the land across the river by building a pair of Ferry Landings.