Can I react with a hidden card at one battlefield when a showdown starts at a different battlefield?

Ruling: You can play a hidden card as a Reaction whenever you have priority to play a Reaction, but hidden cards can only target the battlefield they are hidden at.

Nuances:

• You can only hide cards at battlefields you control, one per battlefield
• You can only play hidden cards starting the turn after you hid them
• Hidden cards cannot be played at your base, only at battlefields