Depending on where you decide to build, some of the waterborne invasion forces might land on an unoccupied island. Don't count on enemy soldiers meekly going away too often, though. Eventually, you'll probably expand your city onto some of the islands. You can place Forts on island settlements to guard them, or build Transport Wharves to shuttle troops around. The latter can take a lot of time, so you'll have to figure out a layout that will allow you to respond to threats in a timely manner.
Invasions begin within the first few years of the mission.
You should try to bring your military to maximum strength as soon as possible, because the Kushites will attack in large numbers. A strong naval presence will be necessary. Always remember to target enemy transport ships first. Requests for goods such as game meat, debens, linen, and barley will demand your attention. Fulfilling these requests will improve your Kingdom Rating, create new trade routes, and sometimes earn you gifts of valuable copper.