Kilburn - this is where you start this quest
City entrance
The beginning of the dungeon
The bridge to the city
The descent to level 2
The beginning of level 2
Montbart
After you have determined who the leader of the Highwaymen, from the "Search for Kilburn" quest(1) is, you decide to set out to find him yourselves. The entrance to the hangout is near (2) the tower itself. You simply need to go Northwards and then to the East. Once you encounter two ghouls, this means that the entrance to the Lost City is close.
The entrance to the Lost City
As you enter, you need to be very well prepared. A big supply of potions and scrolls is recommended here. It is also imperative that you have , on you, scrolls that protect against paralysis, because there are banshees inside. The minimum suggested party level is 15, but it is difficult then anyways.
Entrance to the castle part
Once you enter(3) you should quickly locate the bridge to the castle(4). Kill the two guards and open the gate with the key that they drop. Try to go in the direction in which you walked over the bridge, as often as you can, and you will reach a descent (5).
At the next level(6) trifling is over. Here, you can even be attacked by a dozen of enemies, at a time.
Montbard, is not too tough, but you need to keep your vigil
This is where Montbard is(7). At the first fork, you need to open the door, with the key that you find at this level.
Once you find him, there is no point in descending any further. You can exit through the door that the Southern corridor leads up to, and take a turn at the first fork. This way, you will get to Seahaven.
Then, you only need to return to Morgan in the castle and the quest will be over.