how to build a hidden door
You can build a hidden door by concealing it behind something that does not alert to much attention. A good example would be the classic bookcase. For other ideas you can look here: http://www.oddee.com/item_97712.aspx.
To build a hidden door you will have to remove part of the wall. You build a room behind the hole and then you can cover the hole with a fake bookshelf.