"It's bewitched to look like the sky outside, I read about it in Hogwarts: A History." -- Hermione Granger (PS7)
Vast chamber with an enchanted ceiling that mirrors the sky outside. Four long tables for the four houses. Closest to the doors from the Entrance Hall is Slytherin, then Ravenclaw, Hufflepuff, and next to the far wall, Gryffindor (GF12). The teachers sit at the High Table, a table on a raised platform at the front of the room. Hundreds of floating candles provide light for the room.
A door behind the staff table allows a person to enter the Great Hall from outside without going through the main entrance (Hagrid enters this way to the welcoming feast in GF12); it isn’t clear if this is the same door as the one leading into the antechamber.
There are windows into this room from the lawn outside (CS5).
During the climactic Battle of Hogwarts, the fighting ended up in the Great Hall. The last combatants were:
Bellatrix fighting Hermione, Ginny, and Luna
Voldemort fighting McGonagall, Slughorn, and Kingsley
Molly Weasley took over fighting Bellatrix with her famous battle cry “NOT MY DAUGHTER, YOU BITCH!” and killed her. When Voldemort saw this his anger blasted his three adversaries away, but Harry stepped out from under the Invisibility Cloak and took on Voldemort and defeated him.
The Great Hall acted as temporary morgue and triage station, with wounded fighters begin treated on the platform at the head of the Hall and the dead laid out on the main floor (DH34, 36).
There is a smaller antechamber connected to the Great Hall, door behind the teachers’ table, with a fireplace and many portraits (including that of the Fat Lady’s friend Violet) off the Great Hall (GF17)
Arrangement of tables and the door to the antechamber in the Great Hall:
"Harry, Ron, and Hermione walked past the Slytherins, the Ravenclaws, and the Hufflepuffs, and sat down with the rest of the Gryffindors at the far side of the Hall..." (GF12)
"Harry saw Viktor Krum rise from the Slytherin table and slouch up toward Dumbledore; he turned right, walked along the staff table, and disappeared through the door into the next chamber." (GF16)
"Harry got to his feet, trod on the hem of his robes, and stumbled slightly. He set off up the gap between the Gryffindor and Hufflepuff tables." (GF16)
note: in PS7, the table positions differ slightly. When Harry stands in front of the High Table, facing the students, he sees the Hufflepuff table on the right, Ravenclaw as second from the left, and Gryffindor on the far left. That makes the order in 1991 (starting from the doors): Hufflepuff, Slytherin, Ravenclaw, and Gryffindor. Apparently they changed places by 1994.
Details of the Entrance Hall:
"Hermione seized Harry's arm and dragged him across the [Entrance] hall to the door of a broom closet..." (PA21)
"Harry could hear the drone of hundreds of voices from the doorway to the right...but Professor McGonagall showed the first years into a small, empty chamber off the hall." (PS7) (the Great Hall is on the right when you come in the front doors)
"Feeling oddly as though his legs had turned to lead, Harry got into line behind a boy with sandy hair, with Ron behind him, and they walked out of the chamber, back across the hall, and through a pair of double doors into the Great Hall." (PS7) (the antechamber is across the hall from the doors to the Great Hall)
"When they spotted Crabbe and Goyle coming out of the Great Hall, Harry and Ron hid quickly behind a suit of armor next to the front door." (CS12)
"By far the hardest part was hiding them in the [broom] closet across the hall." (CS12)
"Instead of going up the marble staircase, Cedric headed for a door to its right. Harry stood listening to him going down the stone steps beyond it, then slowly he started to climb the marble ones." (GF17) (to the Hufflepuff common room)
"She turned left at the bottom of the staircase and hurried toward the door through which Cedric Diggory had gone...He and Ron followed Hermione down a flight of stone steps..." (GF21) (to the kitchens)
The location of the staff room
Harry, Ron and Hermione are in the Entrance Hall. "They turned to go outside, but Snape called them back...[then] he strode off in the direction of the staffroom." (PS16)
Windows in the Great Hall, near the front door:
"'I think the feast's already started,' said Ron, dropping his trunk at the foot of the front steps and crossing quietly to look through a brightly lit window...Harry hurried over and, together, he and Ron peered in at the Great Hall." (CS5)