How do you know where barbarians will go to?

[el_kalkylus]

I want to keep some cities undefended, but don't know which.

Also, I want to buy their triemes/boats, but don't know where to wait with my diplomats.

 

[Smash]

I don't know of any way to predict where they will spawn.

Filling up land so it is within your borders works,but even then they can spawn from a 1 square lake right next to your capitol

Leaders will head for fortresses if present once they are alone.

You can control which square they land on by stationing units on other squares but that only works if see them coming.