It's as the gang are being led by Lando to their surprise confrontation with Vader. They walk down some steps which (since the SE) have a large window behind them - you can see an elevator travelling up the outside of the building through them.

But, when they reach the door and Vader appears, shots of the gang and the stormtroopers behind them don't show the window - just the art-deco style pre-SE wall. It eluded me for a long time because it almost seems like they take a 90 degree turn before the door, but they don't.

DE