So, I'm making a sort of player hideout mod, and I've got the trapdoor to the cell set up, but when I go through the door, I spawn in the void? Then I sort of fall and land a bit away from the ladder inside the hideout. What's going wrong? Is there any way to adjust the place where you end up when you go through a door? Thanks.

Did you set http://www.creationkit.com/Bethesda_Tutorial_World_Hookup up correctly?

Thanks I think that's solved it For some reason the yellow teleport marker was in the void, I didn't even see it there, or know I needed to reposition it for that matter. I'm gonna test it now, thank you!

Yup, that yellow teleport marker is where the player and NPC's spawn when they travel through that door, and is also the point NPC's travel to to 'use' the door and travel outward through it.