Remember, you can dispatch a combination of both warships and transport ships carrying soldiers. Requests for military support come frequently, sometimes while you still have troops abroad. It's a good idea to continue to build ships and Forts while your forces are away. If you can spare them, keep some troops at home in case the Nubians try to conquer your city. Watch out for invasions that come from both land and water.
Usable land is scarce, so plan carefully and consider expanding to the opposite bank of the Nile.