As you climb up the spiral staircase up through the gap in the ceiling, you see nothing but brick all around you, as if you were inside a large chimney - a very tall chimney!
The chimney theory is supported by the fact that when the brick finally runs out, all you can see is smoke all around you. You can barely see the stairs in front of you!
Higher and higher you go, wondering 1) How much higher these stairs can lead and 2) Why you didn't see this from outside.
After what seems like an eternity, the smoke and the staircase vanish and you see yourself outside a massive doorway, 40 feet high, set into a cliff wall.