Here is an alliance event which comes along about every 4 weeks. This is a tough event and will cause massive troop injury. Occurs around 12 hours.
So here is one of the Barbarian camps once they start their invasions. So, what do they do? How do the invasions take place?
In this example, the Barbarians may try to Demolish your alliance's regional structures (mason's guild, stables, trading posts- all of it). If they are not stopped, the levels of all those buildings will go back to level one then when they leave, the alliance would have to upgrade the buildings all over again. Higher levels structures will provide more quality.
The barbarians could deplete resource tiles. This is of not much trouble, just that we cannot gather from respective tiles unless we chase them out. I forget to screenshoot the strength of the invaders. What I could remember is that their camp has about 3millions of them, and when they invade in small groups, most of them are higher tier troops and also faction units, meaning that they are very tough to be taken down.
So this how you score. So the next assault event would be in about 3 weeks, so get ready!