I noticed the double walls too. I stood in the cabin for almost an hour trying to find an entrance somewhere
I doubt the Dimension Door and CTRL J would work. If you try to move somewhere you can't go your character ends up standing next to the object (ie still in the cabin)
Sure would like to know what's behind there though. (maybe it's the legendary longbow that fires an unlimited amount of 5+ arrows. Then again, maybe not
)
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The last arrow of Legolas kindled in the air as it flew,
and plunged burning into the heart of a great wolf-chieftain.
All the others fled. -J.R.R. Tolkien, The Lord of the Rings