The Western Guard Post is the entry to the Horned Hold proper. Regardless of whether players are coming from the Duergar Workshop or the Great Hall, they find that both bridges across the chasm lead to this room.
The setup is this: each of the bridges dead-ends at a door.
The southernmost doors (left-hand side of the map) are "iron-reinforced" and "barred from the inside". Duergar inside the guard post are watching this approach like hawks. They are apparently dead keen on preventing any attacks from their own southern fortress (the one with no external doors).
A little to the north of the Duergar is the bridge to the northern fortress and the outside world. This bridge terminates at an unguarded, unlocked door. Players can come straight from the Duergar Workshop and across this bridge without being detected.
Possibly it's a secret bridge. Possibly the Duergar didn't think that anyone would dare attack the western fortress without first neutralising the southern one. Possibly Duergar just aren't very good at counting past one. Certainly the module gives no reason why the dwarves are focusing an ungodly amount of firepower (including two semi-sentient animated crossbows) on one bridge while completely ignoring the other.
However you run it, though, this encounter is focused on doors.
Players coming up the guarded bridge are in for a world of hurt. The Duergar, watching through arrow-slits, are able to deploy both damage and debuffs against targets exposed on the walkway while gaining superior cover against return fire. The module offers some alternate combat resolution, in the event that players have crossed the bridge "unseen or in disguise", which is a nice change, with characters able to bluff guards, force the door or pick the lock.
If players get through the door or, more sensibly, have used the unguarded bridge, the Duergar are going to want to restrict player movement. The unconventional setup of this area really limits both line of sight and range, and one Duergar standing in a doorway can effectively hold up an entire group of players. The module offers no rules on barring or barricading the interior doorways but if a DM lets either Duergar or players get busy with the doors it can turn the battle into a doorway-based tug-of-war.
It's a shame there's no doorway rules, actually, because controlling these doors is a key part of the fight. Like the last couple of encounters, the Duergar here are more than willing to run for help, and it's a particularly difficult area to catch fleeing dwarves before they get away and alert their friends - who include two elites and a level 8 brute.
By the time my players got through the portcullis, the workshop, and then the numerous doors of this room, they were willing to specifically request that there be no more encounters based around barring or breaking doors.