Can spells and units share the facedown zone on a battlefield, and what happens to a facedown card when you lose control of a battlefield?

Ruling: Spells and units share the same facedown zone, so you can have either a single hidden unit OR a single hidden spell per battlefield, but never both. If you don't have any units at a battlefield, you lose control of that battlefield.

Nuances:

• You can respond to an ability that would remove your last face-up unit by playing the facedown card before losing control of the battlefield.