Progo i can not fully understand what you mean, hence i can not really answer you on that roll. 

Tryncaryn can see trough the arrow slot, that there are undead humanoids ammasing in the chamber behind big doors. and the big doors are barred by big iron slab. About the doors: They are made of sturdy wood, but it is wood, can be destroyed. might take some time. 1 hour alone, less with help.

Tryn goes back into his chamber as sneaky as possible with a torch in his hand. “Undead inside and main door is barred,” he lets the others know by way of message. He returns to the northern part but peeks through the arrow slits on the western side before and wonders if these undead are small enough to fit through the slits. Back in the northern part he takes a better look at the door to see if it can be barred from his side and would do that if he is able to do so. Then he would sneak upstairs, “I’m going to look upstairs if I can find an opening to lower a rope or something. The main door we can forget for now.”

(Before actually heading upstairs, he waits for confirmation)

(Progo wants to look through the arrow slots along the east wall to see if there is anything on that side since he has dark vision. Then assuming he didn't find anything, he will draw his bow and aim through the arrow slot on the west side to try and help if he can get a shot.)

Progo when looking trough the arrow slots, can ascertain, that on the east is a mirrored chamber to the one that tryncaryn went in.
From your position it seems like you could fit trough the murder holes on the ceiling - ceiling is 15 feet up from where goraseth stands.

Here i will give you a shot of my perspective with added some ghouls. This perspective give you knowledge of what you have all seen to this moment. Additionally when tryn was looking at zombies he could at an angle see more of them, hard to ascertain how much. but the groans also indicates more of them. Now that you are not so in the open space, the stench of dead and rotten bodies starts to irritate. There was a hint of it earlier, but it seem that fireworks of Tharanu drawn undeads closer and amassed stink is hard to bear. /these dotted lines show placement of murder holes, but it is not relevant here :P/

(is it possible for Tryn to lock/bar the door next to him? He would rather not have the undead come in through there when he goes up because the easiest way for him to escape would be through the way he came in and he doesn't want to find his retreat blocked by an army of ghouls.)

well, if you think of some way then yes :) doors open outward from A2.

(Ok, I thought that since A2 looks like a room that would remain operational even when the main door has been broken through (since some ballistae target the corridor leading to their entrance), it would stand to reason that it can be barred quickly and easily from the inside.)

Observing the door, Tryn notices that it opens outwards, even though it does seem counter-intuitive to him. This makes it difficult to make sure that it remains closed. He could install an Alarm but that would take ten minutes. After a few moments of consideration, and learning about the murder holes, he decides to take the risk and goes upstairs, leaving the door closed but unlocked and unobstructed. 

"Wait a second, Tryn. Let's all climb up there. I can levitate one of you up, the strongest. How about Goraseth or Progo? Then you can throw a rope down and help us all climb up. Who wants to fly up to the murder holes?"

((ooc do I understand correctly that the room Tryn is in is possibly connected to the murderholes? So we'd be all together again?))

“I will go first” says Goraseth without hesitation, making sure he has a long enough rope he place himself under the hole - ready for the magic

Gorsety might try to Reach and pułk himeself up witaj dc15 athletics check, or be helped by spell. I will rule that tryn arrives upstairs around the same time as gor. 

Goraseth emerges with mostly dark chamber, northern part is illuminated by sunlight coming in from arrow slits (it looks rather bad in vtt). good news is that Tryncaryn arrives at the same time with a torch. Both of them quickly realize they are not alone. Whole room is webbed heavily, most thick part is marked on the map (goraseth is on the center of it) it is heavy webbing between 4 columns in the room, it is considered difficult terrain. cauldrons are on wheeled carts, as they have been used to roll to the fireplace to heat the oil and then back to murder holes to pour on enemies below. 
Goraseth and Tryn both see a giant spider on the ground. and hear another one in the corner (around A12). So lets start combat. 

1.My beloved team of heroes goes first (rest of the party down below do act in rounds as well)
2. Giant SPIDERS!

“Goraseth! Throw me the rope! I can fix it so the other climb up. I think that fat spider is coming for you!”

Goraseth nods at Tryn and throws the rope to him (free action?), he then swifltly cast his divine shield in order to blocks all incoming attacks while his camarades are making their way up. He then makes his way with difficulty toward the large monster and thrust his weapon at it. "Ah ah, finally some action..."

Bonus action: Shield of Faith (AC is now 21)
Action: Attack the big hairy spider 14, for 10 of damage

guys i just thought of something, i will post updated map, and will let you two revise your turn on account of new information. i initially rolled stealth for the rest of t he spider, but that was before i placed tryn there, and tryn comes in with torch illuminating most of the place, hence unallowing spiders to hide... . Sorry for that.

Tharanau waits for Tryn's shout out that the rope is fixed, then ushers Progo and Rayce up the rope before him, giving them a leg up and holding the rope steady for an easier climb. Tharanau will be the last to climb up the rope. 
If he has an action left, he will shoot a Ray of Frost at the beast (23 to hit for 5 dmg)

OK....changing actions due to the high number of spiders !!!

Goraseth sees Tryn and the four massive creatures on his way. "No Tryn, stay out of range ! I'm doing it."
He then secure the rope and throws it down shouting "Big Hairy Spiders ! Four, maybe Five !" to his companions below. He then swifltly cast his divine shield in order to blocks all incoming attacks and makes his way with difficulty toward one of the large monster "Ah ah, finally some action..."

Bonus action: Shield of Faith (AC is now 21)
Action: Attaching Rope
Mouvement to E3

"Ladies first, and remember webs are flammable."

Tryn nods to Goraseth and hops up the last step before launching himself at the spider in the South. In his estimation the fat spiders might have difficulties to manoeuvre the little space that there is and he decides to attack the one in the North, so that Goraseth only would have to face three of them at once. 

Move: to I4
Action: Attack @spider 2 (orange) with quarterstaff one-handed: hit 17 for 7 damage
Bonus: Attack @spider 2 (orange) with unarmed strike: hit 18 for 7 damage

Rayce will climb up the rope and look around with devil's sight. If the area is clear she will move towards A12 with what ever movement she has left.

She will also cast misty visions to simulate a group of soldiers coming up behind her and filling the area from E6 to C8 read to fight. She hopes to make the spiders waste some of their attacks on the illusions. The illusions will move out of the way of any pcs going through but look menacing for the spiders.

since it is not high i will allow to climb without check, and Tharanaus giving a hand as free action, as i believe it would succed without it. 
Progo your turn is still up, and yest you can shoot either orange or black spider.
Rayce you might want to pick different place, since when you will go up you will see a spider exactly at A12.