How does the fortress work?

The fortress building affects the level of durability your city has when attacked. Upgrading it will increase the durability meaning it needs to be hit more times before being destroyed.

You can also use this building to repair your durability quickly as opposed to letting it repaid itself slowly. Upgrades also add small improvements to your attack and defense when being attacked.

There is also the option to manage your garrison which means to set the order that your legions will defend or also to set your legions to not defend you. Taking them out of defense will save you some troop losses (although not all) when you are attacked. But will mean you are very likely to lose.