Can units be played to a battlefield mid-showdown, specifically: (1) Can an attacker play a unit revealed from Stacked Deck to the battlefield they're attacking? (2) Can a defender play a unit (like Shen) to a contested battlefield when reacting?

Ruling: You must control a battlefield to play units to it. An attacker cannot play units to a battlefield they're attacking (they don't control it yet). A defender can play units to a battlefield they control when defending.

Sequence:

• When attacking a contested battlefield, the attacker does not have control
• The attacker can only play units to their base or other battlefields they control
• The defender retains control during the showdown and can play units there in defense

Nuances:

• If a unit is revealed via Stacked Deck during an attack, the attacker cannot play it to the battlefield being attacked
• The defender can play reactive units (like Shen) to help garrison during a showdown