Clearly things are not going to go well in this chapter. Our party was chased into the mountain by a tentacled monster and ended up finding the tomb of Balin. And that tomb happens to be surrounded by corpses of slain dwarves. From the written record left behind we learn that the Balin and his folk fell within five years of establishing themselves within Moria. The Watcher in the Water grabbed for Frodo and then when it failed in the capture proceeded to tear down the entrance, and the text indicates further danger and doom ahead.
But now Gandalf knows where they are, and they have the book to take with them. Yet the doom doom of drum beats reaches them, shaking the walls.
Hobbits surprise the men with their fierceness in the ensuing battle, with even gentle Sam taking down an orc on his own. Frodo also proves to be a hobbit of surprises after he survives a felling blow from a orc chieftain, courtesy of Bilbo's mithril armor.
What is not said in the story here, but can be found through supplementary materials, is the nature of the unknown being that makes Gandalf state "I have done all that I could. But I have met my match, and have nearly been destroyed.!" The same creature, a balrog, fills Legolas with such terror that he cannot fire his arrow. A large form of smoke and fire, twice the height of a man or elf, with wings and whip. A balrog is truly the match of Gandalf, being of the same basic stock, that of the Maiar only fallen, and a part of the greater mythology of Middle Earth. There's perhaps even a level of symbolism for the whole tale in this encounter, that even when the light prevails it may be wounded by the dark... something that definitely will prove true for Frodo as this journey continues.
Like everything else in the film, there's a little bit of compression here for story purposes. What stands out most immediately is the dropping of an item down a well into the deeps occurs within the same room as Balin's Tomb and is followed immediately by the threat of orcs and their cave troll, rather than the significant spread of time and location in the story. Everyone gets in on the melee action, with some especially hobbit like resourcefulness in both improvised weapons and traditional.
Where things really differ in the tighter details is the unerring focus with which the cave troll targets Frodo, the respite from battle within the guardroom before running to the bridge, and the independent threat of the balrog. The targeted threat to Frodo makes sense as dark things are routinely drawn to the Ring. The balrog as an independent threat, one that even the marauding orcs are terrified of makes it an even greater unseen danger.
The architecture is grandiose, if bordering a bit on "who designed this thing?" The bridge is totally a narrow edifice over a deep gorge, a defensive structure itself. The rest owes some of its state to decay, and some to an adventurous lack of safety consideration.
Gandalf's fall felt like a betrayal to theater-goers unfamiliar with the books. Of course, in a post "your favorite character will likely die" Game of Thrones audience it would likely take it more in stride.
I was totally one of those smug assholes who knew he was coming back.