If I use Tideturner with only one unit on a battlefield that has a hidden card, does the hidden card die during the swap or does it stay?

Ruling: The hidden card will stay. You don't lose control of the battlefield during the Tideturner swap.

Sequence:

• The unit movement from Tideturner is treated as simultaneous
• There is no window where you don't have units at that battlefield
• Cleanup happens after Tideturner resolves
• At cleanup, you still control a unit at that battlefield, so you maintain control and keep the hidden card

Nuances:

• Think of the move as simultaneous for all intents and purposes, preventing any loss of battlefield control during the swap