Rinn quickly turns to Silenus, "Get down. It's coming."

Silenus looks from Rinn to the bridge and back into the entry way they've gone into. He pushes further in, opening any door/gate in their way so they have more cover from the dragon.

Silenus, you're in the main hold gate alcove, trying to get the hinges to move, when you hear Rinn, "Get down. It's coming."

You turn to look behind you out the path to the bridge. You don't see Rinn at all.

What you do see, is the white dragon's head suddenly appear from the side of the bridge, facing away from you and releasing a torrent of ice breath upon the mercs on the bridge. 

The mercs on the bridge are caught unprepared and by surprise (  CON 11 Cold dmg 83 )

You see the three mercs are ice as the dragon launches itself across the bridge at the archers in the gatehouse tower. (fly 80 ft)

Rinn, it looks like the dragon is occupied. It doesn't look like Silenus will get through that door. You saw the dragon slink out of the rubble in the fallen corner. 

(The door is immovable then so no DC needed?)

(( Correct. It's firmly set in place. ))

(How far are they from where they left the cart?)

"Looks like we're stuck here dude," Silenus says to Rinn, gesturing at the doorway. "Did you see where gnarly draco came from? Maybe there's another way there."

Rinn nods and sends a message to Silenus, "Leave it. Let's check if there is another way through the rubble while it takes care of the mercenaries."

Rinn's Perception if needed: 18

(( The cart is across the bridge, through the gatehouse (large plaza and towers), and down the path out of sight of the gatehouse. ))

(( The collapsed and crumbling southwest corner of the hold would be easy to slip through, you (Rinn) saw the dragon slip out of it without issue. The hold is reportedly old and deep, surely you could find much more than the dragon's nest in there, maybe even an alternative way out. ))

Silenus nods at Rinn and instead scrambles through the crumbling collapsed corner of the stronghold while the dragon is distracted. (Athletics if needed: 17  )

You are able to get to and through the large hole that is the crumbled corner of the hold. 

Once inside, you find the layout much like Axeholm only oriented to face south while Axeholm faced north at least in this first floor. There is ice everywhere creating large jagged spikes along the ceiling and walls. The chamber is at least as big as Axeholm's main chamber was if not bigger. You can see it also once had a mezzanine level like Axeholm but it has been torn apart. The doors for that level are just present on the wall with no surface to stand on. 

The doors on this level are barricaded behind thick sheets of ice. 

There are lower levels and the staircase on the north wall of the keep is open.

There is an upper level, but no obvious way to reach it 

((The staircase on the north wall - that's a staircase going down or up?))

Going down 

((Hmm, okay. The mezzanine doorway with no mezzanine, is that wall iced up too? Is the doorway open? How high up is it? Thinking of climbing up))

There is jagged ice up the wall. It's about eighty feet to what is left of the upper staircase. 

The Axeholm staircase was Solid stone. What is left of this upper staircase looks similar but covered in jagged ice. The mezzanine level is just gone. With some good climbing and a couple of dangerous leaps, you could probably make it up that wall and over to the upper staircase. (( Athletics DC 15 and Acrobatics DC 18 - two separate rolls ))

Silenus looks at Rinn and then at the icewall. 

"What do you think?" Silenus asks. "I don't see any other way up, dude."

He pulls out his climber's kit and checks the pitons and harness.

"Unless you've any other ideas, I'm giving it a go."

(( With climber's kit & rope, Athletics DC 12 and Acrobatics DC 13 ))

Rinn shrugs and uses his mage hand to carry the grappling hook up and secure it. He offers Silenus the rope, "I'll keep an eye out while you climb." (Perception: 17)