They are often seen as gateways to other worlds.  

They can also serve as protective devices. Some ways to guard the home:

Paint the door blue Place two crossed needles under the doormat Sprinkle Mustard Seed on the doorsill Ferns, Lilies, Marigolds or Junipers grown in pots on the front porch will help protect the house. Fill an old sock with salt, sage, mullein, tansy or any other protective herb and bury it beneath the front porch to keep ghosts away from the home. Hanging a Gourd on the outside of a door will ward off negativity.

The doorway can also be a great place to invite energies into your home.

Five pennies placed beneath the porch will bring money & love to the household. A bit of food buried near the doorway insures that the family won't know hunger. If you wish to see a ghost - at dusk or mid-day stand at any door, in the dark, and look into another room - the door must be half-opened and lay your cheek against the door and peer just past its edges into the other room. To get rid of ghosts, slam the door several times in succession.

• Keys:

  Keys hold many meanings.

They are seen as phallic symbols - representing the masculine. They also represent wisdom, attainment of higher consciousness levels, & protection. Some Spells relating to keys:

You can wear a key to obtain wisdom Place a key under your mattress to assist with sexual impotence. Obtain as many keys as you have doors in your house.  Move throughout your house to each door and touch it with a key saying, "Lock out thieves in the night, Lock out thieves in the light, Lock out thieves out of sight".  Now tie all the keys with a ribbon and hang them over your front door. A gold key guards against the evil eye. Three keys on a chain bring wealth, health & love. A single key carried will bring good luck, but only if it Doesn't' fit any of your locks. A key placed in the baby's cradle will protect it from being stolen by fairies.

• Windows:

Windows are doors without keys. Pentagrams can be carved or chalked onto windows for protection. Hanging a thick white curtain protects from negativity. Drive away evil spirits by placing small white pebbles, seashells, a large red tomato or a ball of green grass on the window sill. Washing your windows with ammonia or vinegar invites the power of the wind to bless your home. Stained glass windows can drive away evil spirits. Round Windows are considered protective. To improve luck - sprinkle some salt on your window sill.