Can you initiate a challenge with no characters? I have Horseback Archers in my hand and no characters in play. I want to put him into play during a military challenge according to his ability, but the military challenge must already be initiated, correct? So can I initiate a military challenge with no characters declared as participating and then put Horseback Archers into play to participate in the challenge?
No, you must initiate a challenge by declaring attackers, so you can't put in Horseback Archers as an attacker since you wouldn't have initiated a challenge. You can do that as a defender however.