2nd edition rules say: "If the ambush is not blocked then the opposing player must select one card that is marked with the ‘ambush’ symbol (as shown here) from either his hand or his reserve and place it back on his empire deck. If a player has to lose a card from a siege then he must adjust his strength on the siege strength track according to the military strength of the ambushed card."
What does it mean?
If the ambush is not blocked, the opposing player can select one card from Siege Card space?
Like water spilled on the ground, which cannot be recovered, so we must die. But God does not take away life; instead, he devises ways so that a banished person may not remain estranged from him. 2 Sam 14:14
Ignore that. There was a proposed rule amendment to accommodate ambushing siege piles that was later removed. However, that explanatory rule was not removed prior to printing.