It would look like a normal door at first, but then when you try to open it, it will be a tall block so then you can't enter.

You could change it so it would only be real for you, or for you and citizens if your a mayor. Or you could set as a prank so it would work for everyone except a chosen couple of players. Or mabye it would work for you at first, then when you open it again, it would be a fake. It would work as a cool trap.