Remote switches can add significant variety to map design.
A SWITCH (subset of a door) can also behave as a remote trigger. Simplest way to do a switch break it into two parts, what you see on the wall, and what you activate. make a keypad brush texture it. in front of it, create an invisible brush and bind it to a switch(subset of the door object).
Link it to the door you want to open. done. (You should cancel the activation trigger on the door, so the only activation that can work is from the switch) Now switches in the actual game are more complicated, they send messages and change the textures that you see on the keypad. That's more advanced stuff. If you deconstruct one of the switches from the samples you can figure it out.
msg door34 trigger;
As you may imagine there is alot more to this topic.
This is classic TOMBRAIDER, pull the lever over here, opens the door over there.
Except these days, we as gamers expect a little more sensibility to our puzzles. They can't be haphazard, they should be believable within the context of the level.