Marshes are the best natural defense tile in the game. They're stronger than a fort on a hill, maybe even stronger than a citadel. You just need a seige unit to put behind the marsh. Evemy units are forced to stop on the marsh because it's rough terrain, however they still get the open terrain penalty there. And the seige unit can still hit things behind the marsh, not getting blocked by it the way they get blocked by forests or hills.
In my current game I have a city positioned behind a marsh, with mountains and the coast blocking off any other way of approach. This city is completely invincible to enemy attack! I wish I could put a marsh like that in front of every city.
In my current game I have a city positioned behind a marsh, with mountains and the coast blocking off any other way of approach. This city is completely invincible to enemy attack! I wish I could put a marsh like that in front of every city.