In Harry Potter and the Philosopher's Stone there is a part where Professor Quirrell informed everyone about the troll in the dungeon at Hogwarts and probably fakes his faint. Then Dumbledore tells everyone to be quiet and tells the prefects to take the students of their own house to their own common rooms.
But the common room of Slytherin is the dungeon. So why did Dumbledore tell the Slytherins to go there?
This is the description of the dungeons leading to the Slytherin common room in Chamber of Secrets:
"The Slytherins always come up to breakfast from over there," said Ron, nodding at the entrance to the Dungeons. The words had barely left his mouth when a girl with long curly hair emerged from the entrance.
"Excuse me," said Ron, hurrying up to her, "we've forgotten the way to our common room."
"I beg your pardon?" said the girl stiffly. "Our common room? I'm a Ravenclaw."
She walked away, looking suspiciously back at them.
Harry and Ron hurried down the stone steps into the darkness, their footsteps echoing particularly loudly as Crabbe's and Goyle's huge feet hit the floor, feeling that this wasn't going to be as easy as they had hoped.
The labyrinthine passages were deserted. They walked deeper and deeper under the school, constantly checking their watches to see how much time they had left. After a quarter of an hour, just when they were getting desperate, they heard a sudden movement ahead.
From the last paragraph it is clear that the Dungeons are simply massive. While it might have been safer to keep the students in the Great Hall, Dumbledore may felt that there was little chance the Slytherins would actually run into the troll if they just went straight to their common room.
The Slytherin common room is in the dungeons at Hogwarts, but isn't the entire dungeon. Several other rooms and corridors are down there, including (but not limited to) the Potions Classroom, Snape's Office and the many rooms that were traversed to get to the Philosopher's Stone. Presumably the Slytherin prefects took students via the quickest possible route to their common room, while maintaining caution there was a troll on the loose in the vicinity.
This is an interesting question. Normally all dungeons in a castle are located in a certain area in order to have a more structured control over the prisoners. So, it is logical to question Dumbledore's decision to send the Slytherins in the direction of the attacking troll.
Then again, the Slytherin common room is located well below the castle. It is infact, underground because the windows to their Common room are below the Lake's surface. They can see the life inside the lake.
This makes me wonder if they had a different path to go their common rooms which circumvented the regular dungeons. Also, the design of a dormitory would be different from a dungeon so presumably these were created specially.
