Doors are essentially walls that can be toggled to be opened or closed. They were added in the Community Experience update. There are five tiers of doors: wooden, stone, golden, diamond, and amethyst. Doors can only be used by the owner and his teammates (with a totem).
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 to have the 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.)