Do we see any signs of traffic or tracks into or out of the castle that might indicate an opening?

Make a Perception or Investigation check!

Investigation - 11

There's some sort of sandy path or drainage area or something near the broadly collapsed area along the north face of the castle. You'd have to get closer to look for openings or tracks.

Let’s continue on till we can see if that collapsed spot over may be a way in 

Moving further west and carefully approaching the sandy area, the party can see that Jurgen may be on to something. There are many goblinoid footprints in the sand leading in and out of the collapsed area on the north side of the ruins. Further, they spot a sturdy flap of canvas hanging in a shadowed cavity within the ruined area.

Quinn's questions for Bryony still hang in the air:

“Where do they have Kimberly and this dwarf? Are they shackled? Locked up in a cell?

Jurgen excitedly, but hushly says while pointing, "That's our entry point there."

“I suspected this area is somewhat adjacent to what I thought to be a cell of some sort.”

Syzygy  paces back and forth. It’s not the getting inside he’s worried about. 

Quinn leans on his axe to steady himself. He inhales deeply, spits blood and readies himself.

"If we're doing his, we should hurry."

Most of you still seriously injured (everyone having received only the benefits recently of a single short rest), the party prepares to leave the cover of the woods and enter the collapsed area on the north side of the ruined castle, where it appears a well-trafficked entrance may lie concealed behind a flap of canvas.

Any further preparation? What is your marching order? Are you bursting in or using Stealth? (Roll for Stealth, if so.)

Syzygy feels a slight breeze, calming, encouraging, as if some force beyond his comprehension exhaled a breath. “The time is now.” He says pulls his cloak further over his head and dashes for the flap of canvas concealed. 
stealth: 21 

Syzygy quietly pushes ahead into the rubble, through the dusty canvas flap and into the ruins. (Unless the rest of the party has other business first, well presume everyone follows Syzygy inside.

The tower within has almost completely collapsed, although the ground floor still has a little open space. Rotting crates and ancient barrels show that provisions were once stored here. A heavy curtain obscures the crumbling area to the south, and an intact door leads east.

All is quiet. 

Jurgen whispers and says "Which way to where they were being held?"

Quinn lets the others lead and hangs back standing watch. He readies his longbow and scans the area for threats or any movement out of the ordinary.

Perception check:16

He then follows quietly once everyone safely makes it inside. Once inside, he swaps out his longbow for his battle axe and shield.

Stealth check: 25 

Inside the ruined tower, amidst the ancient, rotten barrels and crumbling crates, Bryony, Quinn, Jurgen, and Syzygy prepare themselves for whatever's to come.

What do you do next?

“To the east,” Syzygy points “was where I last saw Kimberley. I don’t think we should have issue with traps, so said a goblin earlier,” he says unsure “Groll looks packed with muscle but let’s hope his henchmen remain distracted for the while. We fight fast and ferocious.” 

first syzygy would take a peak behind the curtain to the south, if someone on the other side he will communicate this with the others. If empty he will, actually take the most direct path with the door to the west. 

Perception 10 

Looking through the curtain south, Syzygy sees no one – that room is empty of inhabitants. The coast appears clear.
(Which way does he head next??)

Jurgen suggests going to the west. 

Jurgen listens at the door, then if no sound, slowly, carefully opens the west door and peaks inside.

Jurgen and Syzygy peek through the door to the west of the ruined hall. It's almost pitch dark inside, with just slivers of dim light escaping two sets of heavy curtains blocking a matching pair of archways north. Fortunately, they both have darkvision.

This high, narrow hall looks as if it might’ve been part of a chapel or shrine at one time. Angelic figures are sculpted along the room’s upper reaches, overlooking the floor.  Between the archways is a cracked but ornately carved stone brazier.

Syzygy remembers being escorted by two nervous goblins through this room, as though they were afraid of this space.

(Oh Man. I got my directions mixed up, but its good. I meant to say East)

Jurgen will suggest moving over to the EAST door and listen at the door. Then if no sound, slowly, carefully opens the EAST door and peak inside.