He means on the Mediterranean Defense game, you can station assassins in each of the cities.
Alexandria, Athens, Damascus, Jerusalem etc etc...
Each of this city can hold up to 2 Assassins initially. More can be added by completing the "Install a new Assassin Den" mission for the particular city.
Press Square on the Mediterranean map to station Assassins there.
1. Stationining Assassins will reduce the rate at which your "Assassin Control" drops.
2. Having 5 Assassins stationed in 1 city can help you maintain 100% control for a very long time. This way you don't have to keep doing missions for the city just to maintain control of it.
In Istanbul (the city Ezio is in if you don't know), assuming you control all dens, you have maximum of 12 Assassin Slots.
1. Lets say you have 12 Full Slots of Assassin Apprentices.
2. You station 1 of them to a city in Mediterranean Defense.
3. Now you have 11 filled Assassin slot in Istanbul, so you are free to recruit another Apprentice.
Again, don't confuse "station" with "assign to mission". Assassins assign to mission will return to you in Istanbul after completion/failure. "Station" means putting them in THAT city to maintain control
magnifico explicacion, grazie...