"Maybe the Speaker knows something more about that flying woman." murmurs to himself the dwarf as he makes his way to the watchtower.
Krom then attempts to open the watchtower's door while preparing himself to force it open if necessary. "Let's regroup with Rhogar and free the hostages before checking the rest of the castle for the remaining cultists." says to Chill and Kazri.

Athletics 6 + Guidance 4 if required for opening the door

Rhogar scowls and growls guttural at the cultist guards; and yet, after a moment or two of glowering, he eventually manages to take a breath and calm to the point of straightening out once more. However before further addressing the pair he calls out to the others in the courtyard, "Gorrah has entryway secured. Though we saw no signs of anyone using it recently." With that said, he again focuses on the guard, unlatches a set of rope from his pack, and motions for them to turn around. "We're taking you in. Hands behind your back." He demands and approaches them, ready to bind their hands behind them with just enough slack in between the pair to so that they could at least walk closely but relatively comfortably. "Alright. Down into the courtyard you two. Let' see if we can find some place adequate for you to wait aside." He says as he stepped around meaning to having the cultist lead the way down with him at their back holding an end to their proverbial leash.

Chill checked the rest of the courtyard and seeing no one else joins Krom and the others at the base of the watchtower.  "No need to force it Krom.  I've got the keys here if its locked.  Lets try not to wreck too much stuff so the townsfolk don't have to repair everything." Chill then heads over to the set of double doors to some sort of outside room next to the smashed doors of the castle keep.  "Maybe stuff 'em in here for now.  Let's see whats in here first though."

Seeing that everything is still under control inside the keep, Kazri just keep guard of their back. She keeps tabs on the main entrance for any trouble.

If all went well enough binding and walking the prisoner's down to the courtyard, Rhogar grunted in approval of Chill's suggestion, but would hang out with the prisoners closer to the center, waiting to see what came of the Tabaxi's investigation first. As he waited, he'd side-eyed the other bodies, and then glance over to Kazri, trying to gauge her feelings in the moment...

(@Wolfy: Don't know he'd pick up much, but gonna go with Rhogar's passive if stats need be involved.)

As Chill checks the double doors he suddenly hears a bunch of dogs barking.  Opening the door he finds a kennel.  Several kennel cages with 6 dogs total line the walls of the room.  Additionally there is a sled stored inside.  This looks like a perfect place to keep the prisoners out of the way.

Kazri checks inside the keep and nothing new is going inside the keep.

You put the 2 prisoners into the dog kennel and shut and lock the door behind them.  With them out of the way you have the chance to clear the rest of the keep.  No new soldiers have confronted you and things are very quiet now.

So where to?

And is Rhogar keeping the man over his shoulder or locking him up also?  And what about Runa?

(OOC: I think after a look at the trapdoor leading to the cistern, Rhogar will set his imprisoned guard with the rest, and summon Gorrah to watch them while the group presumably carries on to the tower and the trap door at the base of it.)

(If the group is ok. I am going to wake up Runa, maybe she has more info?)

"Shall we proceed to the trapdoor under the cistern and free the hostages?" asks the dwarf as Chill and Rhogar take care of the prisoners. Noticing Kazri near the uncouncious Runa Krom says "I agree with you, she may have further information, but I think that we should question her later. In the meanwhile, we should bind her too and put her with the rest of the prisoners."

(OOC: As for waking up Runa, I think that we should wait at least we have freed the hostages from the cistern and scouted the rest of the rooms. In the meanwhile we can bind her, remove all her weapons and put her with the prisoners/)

"Agreed Master Krom." Kazri takes some rope from her backpack. (If Runa is still wrapped in vines) With a mental command, she unwraps Runa from her spectral vines. She binds the unconscious warrior then toss her inside of the keep, just so that she's out of sight. "She shall not bother us, for now."

Yeah, I agree, keep them all together locked up in the kennel.  Then head for the trap door and down.

"One can hope. Though it is a shame that it's come to this, and that she yet fell upon the broken path once more." Rhogar admits as the doors are closed once more on the former castle "guards". "But with living comes opportunities. So, depending on how merciful the Speaker and the people of town feel, she may yet have one last shot of redemption when all is said and done... and if not in this life, then mayhaps the next." With all that said, Rhogar followed the others back to the tower, as well as send out a mental alert to Gorrah, just in case the 'rambunctious' Psuedodragon ever got it in her head to set up an ambush out of boredom.

Krom ties Runa up and you release the vines around her.  As you go to pick her up to move her she regains consciousness again.  She struggles briefly but then realizes there is no use.  She glares at you but does not speak and refuses to answer any questions posed to her.  You place her in the kennel with the other cultists and lock the door behind you.

Moving to the watchtower you find the hatch.  Nothing has been seen or heard by Gorrah.

A wooden ladder clings to the west wall, held in place with iron brackets. The ladder descends twelve feet to a hall made of mortared stone lit and heated by four sputtering torches. Extending from the east end of the hall is a water-filled cistern. A small rowboat is tied off near the water’s edge.

To the north is an open doorway that leads to a room.  The room is empty except for four sheet-wrapped bodies lying side by side on the floor.

Stone steps descend into the icy water that partially floods this twenty-foot-wide, arched tunnel, which continues eastward into darkness. The roof arches twelve feet above the water, which is eight feet deep. The rowboat tethered at the water’s edge looks safe to use.

Chill and Krom both have 60 ft darkision and ahead they can see openings in the walls, one to the north and one to the south.

Rhogar can see 120 ft and at the far end some sort of large slab of something blocks the far end 

Krom, touching the holy symbol of Moradin on his shield, chants an hymn of evocation. As soon as the dwarf finishes the hymn, the holy symbol emits a bright and warm light. 
The dwarf kneels down next to the wrapped bodies "I fear that we may have found our hostages..." After trying to determine the cause of death and sharing with the rest of the group his findings, Krom points at the rawboat. "Shall we check out those two openings on the walls before regrouping with the Speaker?"

Medicine 12

@DM Krom's original medicine roll was 13.

"Hmn.... Some of them, perhaps. But that cannot be all of them." Rhogar said, passively examining the bodies from over Krom's shoulder after having followed the dwarf(and others) into the adjoining room. "I'd even wager that more than likely these four were perhaps the more stubborn of the hostages, and made... poor attempts at escape. But be that all as it may, I'd agree. Let's see if we can find the others." He said, before once more return to the docked boat. It's then that he'd kneel down and try to inspect it, but also offer, "Hn. If any desire, I can confer the blessing of flight from Brother Vixix to another as a precaution, if nothing else. In fact, I can do so for two. Meanwhile the rest of us can simply take the boat."

-Boat Inspection: 22. (Includes Guidance.)

Chill entered the room to look around but left the inspection of the bodies to Krom and Rhogar.  He looked for any sign of secret doors or anything else of interest

Perception 19

If nothing is found he'll wander back out and walk to the water's edge and look about trying to see as far as he can and also looking in the water.

"I'd like to fly.  Never done that before.  And if it keeps me from getting wet, well all the better then!" He says

While the others are inspecting the room with the sheet-wrapped bodies, Kazri inspects the hallway for signs of foot traffic. "Some people may not know other way to live their lives. Perhaps I am mistaken, but I hope that it is not too late for Runa to see the light." Kazri responds to Rhogar.

Survival: 14

Inspecting the bodies you see that they are three humans and a halfling and they appear to be frozen solid.  Their apparent injuries look to have been received from melee weapons.  But they are not dressed as you might think servants would be.  But what clearly gives it away is that each of the dead has an amulet similar to the other cultists.  These are not servants, they are deceased members of the Black Swords.

Chill checks the room out but finds nothing of interest.  He then wanders to the water and looking in he can see swollen, half-frozen bodies near the bottom, weighed down by their chain shirts. There are six human corpses and six dwarf corpses in all.

Chill peers further into the darkness but can't see anything else that isn't already described.

Rhogar inspects the boat.  He sees it has two oars and can hold two people plus their gear.  It looks sound and has no damage and floats gently at the edge of the water.

Kazri looks for signs of passage but the hard stone floor gives up no secrets.

Torn as he was about the matter of Runa, Rhogar gives only a noncommittal grunt as he held staff tip against forehead, focusing for a moment before then tapping both Chill and Kazri on the shoulder, sending gentle currents of magical electricity scattering across their form. Immediately afterwards, the duo would feel their bodies grow light and buoyant, and with but a thought could ascend move freely wherever the wish in the air. If either looked down however, they'd see sparks of what appeared to be electricity occasionally flicker out from beneath their feet.

"There. That should last about ten minutes. But whenever possible, do endeavor to avoid lingering over nonsolid ground for too long. Though I doubt there's anything to fear, it is best to be prepared. And feel free to scout ahead while me and Master Krom here get underway in the boat... Which appeared solid enough from my inspection." He turns then to Krom. "Come along, Master smith. There is yet more work to be done yet." With that, the dragonborn climbed into the boat after making sure it was oriented, and take up the oars with a confidence and familiarity of one clearly having been raised around the use of such vehicles.

-Spending 3 Sorcery points and twinned cast (3rd level)Heavenly Grace on Chill and Kazri.