Here's what's going on:
* The Challenge Cup is a special promotional card set: It was released as part of a promotional event, and the cards are designed to look like a "challenge cup" with a door on it.
* The door is a visual representation: It's a clever design to add intrigue and a sense of mystery. The door is not meant to be opened, and there's nothing physically behind it.
* The focus is on the cards inside: The true value of the Challenge Cup set lies in the Pokemon cards it contains. Each Challenge Cup card features a specific Pokemon with special artwork and potentially unique abilities.
So, while the door might make you curious, it's simply a part of the design, and the real excitement lies in the Pokemon cards that are inside the "cup".