Quote Originally Posted by Reynaerde View Post
Q104

If I cast the Symbol spell in a demiplane I created with the Demiplane spell, will it still be there when I return next time? In other words: does the demiplane stay in the same place? What about if I cast Symbol on the door created by the Demiplane spell instead?
Yes the symbol and anything else you put in the demiplane will still be there if when you cast demiplane again you choose to to go to the same demiplane.

I don't believe you can cast it on the door, or if you did it would vanish after an hour. Not totally sure about that.