I wanted to provide some comments on enemy warnings. None of the following are consistent, though.
Sometimes I will be notified if an enemy is passing my buildings, like a random wolf running by but not attacking.
Sometimes I will be notified when a large enemy attacks a gate.
Sometimes an enemy can break two gates and kill a person before I am warned. (One time they were on a third door)
Personally I would like to be notified when a new enemy/group appears on the map, and then again when it actually starts attacking, because:
not all enemies attack
gives the opportunity to attack proactively
minor enemies accumulating on the screen weighs on performance.
The current system only warns you when an enemy comes within a certain range of a hearthling, not of any buildings or walls or fences. So the enemies that can will destroy any gates or doors they come across on their way to your town. Gates and doors seem to especially offend them.
My original comments are my actual experiences. I have had people almost dead before it warned (especially my engineer trying to repair the door they are breaking, as the archers/splash damage harm him).
You should maybe move this to the community feedback section since its more about our experience with the game rather than actual game-play. Just click the edit icon for the topic title, there should be a drop down.