So if one of your castles is under siege, instead of trying to get past the enemy and into your castle, in which case you would lose half your troops, you can instead try and attack the enemy siege camp. This will allow an allies to join you in attacking the enemy. The only thing is, if you do this, it begins the battle with you and all your allies inside the castle, and the enemy siege attack AI completely broken. A portion of the enemies will come at you while most of them stay behind their wooden fortifications.