[GM notes: Analysing the map of Cragmaw Castle]If you look at the map of Cragmaw Castle, you can see three 'zones'. The first zone is at the front of the castle (to the west), where the Goblins and one team of Hobgoblins reside. Noise (for instance from a fight) in one room will alert the monsters in the adjacent rooms.
The second zone consists of the defiled shrine and the room with the Grick. There's an empty store-room between the shrine and the first zone, so unless a fight breaks out right there, the Goblins in the shrine will not notice anything amiss in the other zones.
The third zone is the east side of the castle. There's an empty hallway (running from the side door at the south to the hidden entrance at the north), this is an area that nobody loiters around in. And given the fact that King Grol is a mean bully, I think it's safe to say that the Goblins at the west side of the castle are content to stay there! Again, a battle in this zone will not alert the residents of another zone.
This allows the characters to put in a short rest before continuing to the next zone.
[The adventure]With the party safely hidden in the bushes some distance from the castle, the Rogue went out to scout around the hill on which the castle stands. He is sufficiently stealthy to pass unnoticed, and he manages to identify the path leading up to the fake canvas 'wall' to the north tower. After making sure this is indeed a path into the castle, the Rogue returns to the party and they sneak into the castle.
Using their usual cautious routine, the party decides to check out the east side of the castle first. They pass the door to the king's quarters (I ruled that the king was too busy arguing with the Doppelganger to notice what happens outside of his room) and managed to surprise the Hobgoblins that are on guard to the south of the king's tower. After dispatching them (counting the beds to make sure they got every last one of the Hobgoblins), they doubled back and kicked in the door to the king's room, surprising the occupants.
A battle ensued. The Noble Fighter made sure that the wolf was taken care of first (after all these encounters with wolves, he has acquired an antipathy against them), which further enraged king Grol. However, the combined might of the party proved too much for him. Other party members had kept the Doppelganger occupied, so she could not escape with the map, and soon after Grol was felled, she was vanquished too.
Immediately after, Gundren was released from his bonds -- the first time in a week that he could move around freely. His ordeal had weakened him, but he was thankful to see the party again, and to find his map relatively unscathed. When the party asks him to join them in exploring the rest of the castle, he declines to take an active part: he is too weak, but he is in no shape to travel on his own either. He is given a light crossbow for defense and told to hang back (which he does -- the rest of the session Gundren does not take an active role).
The party takes a short rest before opening the door to the south-east tower, and the fight with the owlbear ensues. They take a beating, having to expend some resources. And they even fail to find the treasure in the room!
Then the party crosses the corridor and enters the kitchen. A chaotic fight ensues, with the Wizard making good use of his Flaming Sphere and the rest whacking Goblins left and right. However, some got away and ran towards their sleeping area, alerting the Goblins who were there. When the party pursued the fleeing Goblins, some of the Goblins in the sleeping area stayed to fight while other fled towards the hallway, alerting the Goblins and Hobgoblins in that area, and so on. In the end, the whole 'zone' was alerted!
The Noble Fighter took the door to the north of the dining area in order to cut off the escape of the fleeing Goblins, putting him in the trapped area. He did not move across the wire so he did not trigger the trap -- but when a passing Goblin saw him standing there, it kicked the wire in, triggering the avalanche on top of the Noble Fighter! When he went in pursuit, he walked into the Hobgoblins, and he was struck down. Luckily for him, backup was not far away and he was healed again by the Cleric.
In the end, only two Goblins managed to escape through the front door, and the rest was put to the sword!
After catching their breath and regrouping (because everybody pursued the Goblins left and right, the party had been spread thin, another factor that made the fights a bit more dangerous), the group then went on towards the Shrine, and slayed the Goblins there. The Grick ambushed them when they went into its room, but it too was soon dispatched.
And so the party cleared Cragmaw Castle. Gundren will want to go to Phandalin and rest up for a bit before the party is ready to tackle Wave Echo Cave! Maybe they'll run some of the errands still waiting for them in Phandalin, maybe they will simply wait for Gundren to recover enough.