Doors are essentially walls that can be toggled to be opened or closed.
Currently, doors are only usable by the owner, making it necessary to have multiple doors in a clan base.[Next release will change; See updates]
A common problem with having multiple doors is that on death players lose their structures--forcing the members to make a new one or patch the hole. Try to have as little doors as possible-- having multiple to one person not only increases the need to fix it when they're gone, but costs significantly more(3x wall price for one door).
Another problem may arise when other players intentionally block doors to keep you from going out. Make sure you have the right equipment to be able to destroy any door. (Any tool will work for wooden, a hammer is needed for stone+ doors. All doors can be slowly broken by Amethyst tools.)
Here is the list of doors currently available in game:
(See also, 'Structures')