When I was playing the stronghold level, I wandered into a hole in the map and if you survived the fall (like I did) you can go to anywhere by walking there by looking at the minimap. After you get there you can hit the "return to last checkpoint" button and it will bypass any gates.

You can so this by going to the left side of the outer ring surrounding the portal and try to find it. Its located around some small water pools and you cant realy see it because of the camera angle (and its next to a wall with the water above it) 

I dont now exactly what block it was but I just know it happened

When you fall you'll need a lot of health to survive but it will look like you are walking on nothing above the sky