Try the side door

Because the main door could end with you facing the dragon, you make your way to the side-door to make sure it is open for you. You also notice dark clouds on the horizon and realize that the dragon will not be out long. He is just getting a snack to tide him through the coming storm.

Upon entering the castle, you quickly look around and notice that other than a small open staircase up to a single room above the entire building was just one massive workspace for the previous mad scientist owner. From the dragon, there is a massive pile of treasure piled high in one corner as well as an area where you can tell he sleeps alot surrounded by rubble, treasure, and some of the failed knights' armaments. Near the nest is the rest of the armor weaponry from failed knights. At the top of the room, you notice the underside of that weird lightning catcher contraption you saw on the roof. To your right you see the massive outer door still open for when the dragon comes back. You finally notice a mechanism similar to the ones you use in your neighbor's barn for keeping out large predators from their livestock by heavily blocking the door.

Will you try to bar the dragon from entering longer, loot and run, go up to the princess, investigate the contraption further, or something else?