If I have hidden cards and play Downwell, can I reveal the hidden cards after Downwell resolves?

Ruling: No, you cannot reveal your hidden cards after Downwell resolves because they will be trashed during the Cleanup step. When Downwell returns all units to hand, the battlefield becomes Uncontrolled, and hidden cards at Uncontrolled battlefields are removed and placed in their owner's Trash.

Sequence:

• Downwell resolves, returning all units and gear to their owners' hands
• Cleanup occurs immediately after resolution
• Battlefields with no units and no Contested status become Uncontrolled
• Hidden cards at Uncontrolled battlefields are removed and placed in their owner's Trash
• This happens before you have any opportunity to play the hidden cards

Nuances:

• Hidden cards are facedown cards at a battlefield and are not considered "units" or "gear" while facedown, so Downwell does not directly affect them
• The removal of hidden cards happens due to the Cleanup process, not due to Downwell's effect itself
• Hidden cards can normally only be played starting the turn after they were hidden (they gain Reaction speed)