This pair of demos, both recorded on ACE difficulty, show how to obtain the yellow key on level 20 (Gytowt Station :-).
This task is much easier if one (or more) of the cloaked Smelters give up a cloak powerup. I consider this the "normal" case, and it is covered in L20YKey. You can still get the key without a cloak, as shown in L20YKnc, but it is considerably more difficult.
Both demos cover the entire sequence from outside the blue door to obtaining the yellow key and through handling of the ambush that awaits you once you emerge from the chamber with (or without :-) the yellow key.
One by one, I take care of the cloaked Smelters under the protective fences, while trying to avoid getting hit by the annoying little red gnats. There is also an energy sucker to contend with.
Then it's time to go after the yellow key, which is behind the door in the "far corner" (from the vantage point of the blue door) and protected by a force field. The switch to remove this force field is down below in the same chamber, behind a grate. The lower area of this room is very well protected and equipped with three 'bot generators. I stay only long enough to trip the switch and get the key. I leave the rest of the 'bots to be blown to bits when I destory the station.
Again, getting down to and tripping the switch is much easier if you are cloaked. Since the 'bots can't see you, you can escape with a minimal amount of damage. If you are not cloaked, you must move quickly, and you are likely to take moderate damage.
Upon emerging from this room, a previously-hidden 'bot generator kicks into action. In the cloaked version of this demo, I remembered to leave a smart mine gift for the first 'bot out of the generator, but I forgot in the non-cloaked demo. This made it only slightly harder.
Then it's on to the yellow door and whatever challenges await there! :-)