If a unit at Reaver's Row retreats when attacked, can Shen be played from hand onto the now-empty battlefield during the showdown?

Ruling: Yes, you can play Shen from hand onto the battlefield even after your unit retreats, because you maintain control of the battlefield until the showdown's final clean-up phase.

Nuances:

• The battlefield remains in your control while being contested by an opponent
• If you declare Shen on the chain as a reaction to Reaver's Row, your units would still be present when Shen enters the battlefield
• You can also wait for the Reaver's Row chain to resolve and then play Shen as part of a new chain