Can I play Shen as a reaction at a battlefield when my opponent moves their unit there to conquer, if I don't have a unit at that battlefield? If yes, would Shen be an attacker or defender?

Ruling: You cannot play a unit at a battlefield unless you are in control of that battlefield. Since you have no unit at the battlefield, you cannot play Shen there as a reaction.

Nuances:

• This rule applies even when reacting to an opponent's conquest attempt