The keypad item is an invaluable item for building defensive structures. This allows a 4 digit pin code to be applied to a vertical fort part, entering the code will make the fort part move between it's vertical and horizontal states turning it into a door.
To fit the keypad onto a fort part, approach a vertical fort part with the keypad and hold Interact Key (Default F/Enter) when the prompt to "Modify" appears onscreen.
When the status bar fills, you will be asked to enter a 4 digit (numerical) code, this is the code you will need to open and close the door of your castle so don't forget!