Me and a friend were attacking a keep with two trebuchets each, no reinforcements turned up and the keep did not change on the map to show it was under attack until we started attacking the inner postern wall, when we tried this again the keep icon on the map changed to show it was under attack whilst we were half way through the outside wall, not sure if my friend started a fight with the patrolling guards but either way need an explanation as to what exactly causes a keep to show up as under attack so I can avoid alerting enemies in the future, not fun busting your way through a farm and a keep with 4 people killing all the guards along the way just to get swarmed on the second flag inside the keep at the last minute.
We've successfully completed this once so far but hope to be more successful with the right knowledge.