Doors are probably one of the things I get asked most about. They really aren't that hard, but explaining them isn't exactly easy because there is -a lot- you can do with them and different ways to go about doing them. 

Types of Doors

door doors are by default open, use resets to close or lock
climb players must have climb or fly to pass this door
immortal only immortals may walk through this door
passproof players may not use the 'pass door' spell to go through this closed door
nodetect players will not see this door (here is where good keywords and hints comes into play :)
pickproof the pick locks skill may not be used to unlock this door
smashproof the skill smash may not be used to pass this door
closed use resets to close the door- addreset door [dir] closed
locked use resets to lock the door, reset also closes the door. addreset door [dir] locked

You may also add descriptions to doors! Type 'desc door [dir] [descrip]'

Setting a Door

When you dig your area, you will automatically create "openings" from the room you are in and the new rvnum. Listed below are the series of commands you need to enter to get the specified door.  Mix and match them as needed to get the type of door you want.   NOTE: You do not need to 'set door type [closed|locked]' since this is handled by the resets. These can be made into doors rather simply.

normal set door [dir] to [vnum]
set door type [type]

This is a normal, run of the mill door that you can open and close.

locked set door [dir] to [vnum]
set door [dir] type door
{set door [dir] type [any other type of door except closed or locked]}
set door [dir] key [vnum of key]
addreset door [dir] [closed or locked]

It's also a nice touch if you add the rvnum to the v0 of the key (unless the value is being used)  NOTE: Keys do not have to be type key. Making them type key makes them nosave.

one-way set door [dir] to [vnum] onesided
set door [dir] type door onesided

This is a door you may pass thru one way.. but once you do, you can not go back.

secret set door [dir] to [vnum] {onesided}
set door [dir] type door {onesided}
set door keyword [keyword]

These are the types of door that you have to 'enter'. Good keywords are a must, either in the room description, an extra description in the room or perhaps a clue in a mobprogram. NOTE: Mobs -will- walk thru these doors unless you prevent them somehow.

To clear a door- set door [dir] clear
enter that command twice to clear the door