Unfortunately room is not that big, and torch hits the wall and illuminates Tryncayn as well. 
That little black inverted L on eastern wall is a winch, and on western wall there are doors. Other than that there are stone stairs leading upstair.

Those weird thingies are dwarven balistas. Here is a close up to understand its design, each of those is equipped with 5 balista bolts and looks functional.

Giving up on the idea that he can hide in the shadows if he can't see through the dark himself, Tryn picks up the torch. He first peeks up the stairs and then, down the hallway on the ballista side. Unless he sees any immediate danger, Tryn tries to turn one ballista so that it points to the door but won't put too much effort into it if it doesn't rotate easily. Then, he'll try to move the lever to open the portcullis. 

Tharanau cannot see what's going on and feels bad standing outside not able to help in any way. (Perception 5)

Tryn whispers to the others, "There's a door on the eastern side of this room. I can't imagine it opens to the outside, but it might be worth checking out from the outside."

Since Tryncaryn was looking around the room he could see trough arrow slits on southwest that there is some narrow chamber over there. and from southeast arrow slits he could hear and see some undead (multiple) moving around. (their tokens are not placed yet). When he decides to pull one the lever he meets a lot of resistance since whole machinery is old and rusted.
Tryncaryn please roll 4 Strength(Athletics) checks.

/Others you make a good call on perception checks to look for other entrances, but unfortunately you roll low, oh and balistas do not move around easy, they are not bolted to the floor but are heavy so they would not change position when shooting/

Atheltics: 3, 21, 9, 17 (the second one is a crit and the last one is a portent roll of 16 plus the modification of 1).

Goraseth feels the portcullis shaking and realise Tryn might be doing something. He quickly places his shoulder under a bar and helps lifting the heavy gate. "GGGnnnnnnnnn !!!"

(Atheletic check: Nat 20 !!!!! for a 25 total ! - great timing)

at first you hear a little crank and very little movement. But then Tryn gives it more push and it slowly start to move. For Tryncaryn it feels like he made it loose with his second push, and for Tharanau, Progo and Rayce it looks like Goraseth opened Portculis by himself. Truth is they worked together and managed to lift it over Goraseths head. It stays in that position. There is a blocking mechanism on the lever where Tryncaryn is, it can be loosen to close the portculis immediately

.

"By the goddess, did you see that ? Bahamut's power is flowing in these biceps !". Goraseth feels quite proud of what he achieved - although not achieving anything in reality. He reaches for a torch and ventures behind the gate. "Ler's go my friends, I don't like leaving Tryn alone in there."

The double doors on the south are closed. and you hear echoing groans of multiple sources.

Goraseth tries to push open the doors, in the hope they are not locked or barred on the other side. Any locking system around ?

Communicating with the others, Tryn wonders if there is a mechanism inside that can open the doors. Tryn silently moves south to eavesdrop on any noise that he can hear from there. Clearly, he is well lit, but anything that is there would have noticed that already. He particularly, peeks through the South-Eastern arrow slit, whispering his intentions to Goraseth and the others, "I'm having a peek what's behind those doors. So far it's silent in here. I could go up or through a door. Lots of ballista in here with holes in the walls to shoot through. Quite a bastion, I would say."

(Does he remember if the north-western door in this chamber can be easily locked from the inside? And how looks the one he sees to the west now?)

ooc Note: i just realise We have actually the sending stones, no need to message ;)

Tharanau enters with the others. The sounds make him uneasy. He casts false life on himself (+10 tempHP for a total of 35HP)

Big double doors will only budge a little bit giving sensation that they are barred from the other side. Tryncaryn you had opportunity to check one doors, and they were not locked, the other doors looks similar. there are keyholes in both of them, but the ones you had opportunity to check bu touch were not locked.

“Maybe we should just knock” whispers Goraseth joking. “We are a bit in a pickle here , the door won’t budge and whatever is blocking it might be too heavy for our halfling friend. If only we could all squeeze through this arrow slot.”

Tryn carefully leans through the south-east arrow silt to check what obstructs the door, prepared to dodge any attack on him. 

"Progo suggest disarming those oversized crossbows as you go so Progo can support you through the slots if need be with worry of these getting triggered. " Progo will peer through the opposite walls slots (darkvision) to make sure he wasn't going to shot from behind before returning with bow readied to a slot by Tryn.

Perception: 14

"I don't like Tryn being separated from us. If he gets into trouble we will not be able to help him. We are past the first obstacle and we have made enough noise to rouse whatever defenders this place has. Knocking may not be a bad idea."

She urges Tryn to come back out and everyone to step back out of the kill zone of the crossbows. If they do, she will attack the door repeatedly with eldritch blast to break it down.

"Wait... we were sent here to secure this place for the village as a place to retire if the white dragon shows up. This stronghold  would not be strong if the doors are smashed.". He places his scaly finger on his nose, thinking out loud... "I feel bad but we should ask Tryn to go check what is blocking the doors and see if he can move it. If there are any threats he should run back to us. The other possibility would be to shrink one or two more of us, but I don't think any of us has this kind of magic."

@DM: I did not get a sense of how sturdy are the doors. Would Goraseth assume it can break them easily ?