The non-hobbit faction of the Fellowship rides knowingly with Rohan into Isengard's storm.
The defending team has not fared well in the face of overwhelming odds and little support. But the host turns away from the Fords of Isen and makes way instead to Helms Deep, while Gandalf takes his leave on an unstated errand. On their way they encounter signs of merciless battle, the bodies of slain citizens cut down while they fled and what few decimated companies remain are scattered and leaderless. The surviving civilians who escaped have holed up in the caves of Helm's Deep.
Perhaps unsurprisingly, Gimli totally digs hanging out in caves while Legolas is generally left unsettled. Gimli also turns out to be unexpectedly sneaky... as well as quite eager for battle.
Once the battle is joined it does not ease off, but comes in waves, testing and trying the defenses. As far as sieges go, this is a short one, lasting mere days. The fight is not just man (and dwarf and elf) against orc, but man against man as well, with old cultural grudges flamed into violent action. The assault itself spreads over an impressive footprint, a citadel large enough that thousands can stand in defense of entirely different areas, both inside of the cave networks and from outside the walls.
In the dark shortly before dawn Aragorn offers the horde a chance to parlay, and has the presence to stagger the wild men among the orcs. Seemingly on the winning side, the Uruk-hai decline the opportunity and believe they chased Aragorn away, until the royal heavy hitters burst out into the fray. The hero party comes out in a concerted force and proves quite effective. Then in the eleventh hour Gandalf returns with men thought to be long fallen to the Uruk-hai and proceed to assist in cleaning house.
Looking at the film, there's very little similarity beyond the fact that it's a desperate pitched battle, a culvert is blown open, some thrilling heroics, Gandalf returning with an army, and the kill count competition.
Contextually, these are two entirely different battles, even if their outcomes fundamentally mean the same thing. In the book the battle is an act of major defiance against Saruman's power as well as the reclaiming of Rohan's strength. Their failure would signal the defeat of Rohan in both a symbolic manner. Jackson made the battle a lot less symbolic thanks to putting all of Rohan's eggs in one basket. The assault becomes far more personal, with Wormtongue pushing for the offensive against the vulnerable, and more desperate with the defending forces supplemented with the young and untrained. Adding in the elves helps highlight the waste of war, the dead shining bodies of a favored but diminished race.
At the same time, the grandeur of the conflict is lessened, much of it's harshness offset by physical humor (mostly at Gimli's expense). The humor was something I appreciated far more when I was younger, now it seems to mostly detract from the battle... and skipping the in between scenes highlights how much time is spent on this fight. I still appreciate the dwarf tossing though. On the way to Helm's Deep we get pitched battles of a few dozen heroes against orcs, defending what seems to be a few hundred civilians, and budgets and logistics what they are the scale of Helm's Deep comes nowhere near the massive fortress described. My complaints aside, it is still an impressive battle sequence all around and visually a good match for Alan Lee's painting.
The "loss" of Aragorn strikes me as by and large unnecessary. I suppose it does link well into showing Arwen's Choice and the ramifications, though I can't say I'm fan of the tight focus on a crying face (and I just feel... weird about a father asking is he has his daughter's love, I don't know why, that's probably my own baggage). On the flip side, I wonder if there was an intent to offer a a mirror to Gandalf's fall and return, and his return is appropriately dramatic.
As much as I generally bitch about the added material, the set up here involving Isengard is fantastic (as well as generally visually stunning). Similarly, Galadriel's prophesy/seeing voice over cleanly links the splintered story lines, while building tension. Next to that, I want to commend the development of Theoden, a man faced with hard decisions and few options. Aragorn complains at first of Theoden's proclamations, until he realizes the widsom of Theoden's choices. Strider becomes more actively encouraging of those unlikely to survive, realizing that hope is essential here.