When playing Challenge during a showdown, can you target units at any location or only at the current battlefield?

Ruling: You can target units at any location when playing Challenge. The only targeting limitations are those written on the card itself and the rule that cards played from hidden can only target cards at the same battlefield.

Nuances:

• Challenge can be played as an action during your enemy's turn and target any units, including units at bases
• The restriction to the current battlefield only applies when playing cards from hidden