In the Macedonian town of Agee/Emathia/Edessa/Pella which are on the coast why cant the garrisons at home be kept on auto defence so when the Tiremes arrive they can attack it while the armies are out in the north/West/East conquering as per the game

And what would happen if the enemy is far more powerful? You'd lose the battle AND the town due the missing defenders. You get the alert, the rest is upon you to decide.

well, both comments I believe can hold true, you do get the alarm and yes the rest is up to you, but in other strategy games, whether RTS or turn based units can have stances, they can be set to guard, aggressive, etc which is also good if you don't want to worry about everything, so you just leave a sizable force in a Garrison and they are set on guard, and they guard against all enemy raids, and you continue to get alarms as long as the battle continues, and when a unit either yours or the enemy's is killed you get another alarm, and you know how many units you have so when the number of alarms become more than you units you know that you have to turn you attention to that area...