Well, did Tolkien get it correct? I know he was a medieval scholar. I have the book. I could read about the battle tonight, but I've never heard of LoTR being used as a historical reference to medieval battles. In the movie, there is a ghost army of previously fallen warriors that comes and saves the day. Is that in the book? What is the correct effectiveness of a ghost army?
Obviously (I hope) I'm not trying to use it as a historical reference. It's a fantasy book, no matter how carefully constructed. It has magic. Furthermore, Tolkien could, of course, not get into the technicalities of a battle since he had a story to tell. But we do see Eomer, for instance, leading a furious charge against the enemies on the southern part of the Pelennor, after finding the apparently dead Eowyn (and this is where we get the 'Ride to ruin, and the worlds ending' and 'Death' thingies). His men drive great wedges into the enemy formations, but then they lose momentum and get bogged down, since there's still a lot of the Mordor buggers left and the swan knights of Dol Amroth can't come to their aid immediately as new enemy forces march up to prevent just that. Eomer then dismounts and decides to try and form a shieldwall to fight on foot till the bitter end. So, like ancient cavalry wasn't as overpowered as in RTW, so Rohan's cavalry could not sweep away an entire army as it did in the movie.

The Army of the Dead in the book is nothing like that in the movie. It's effectiveness is in the immense fear it inspires: when Aragorn and his company ride through Gondor with the dead following, people everywhere lock themselves in their houses. The dead are used at Pelargir to scare away the crews of the black fleet of the Corsairs of Umbar (with many of them simply jumping into the river) and are then released by Aragorn, who subsequently mans the fleet with a few thousand Gondorians and sets sail for Minas Tirith.

lets face it how many sieges of massively defended fortresses end as fast as the one in Return of the king? ... One minute, Orcs are invincable, the next they are pathetic
Most real-life sieges don't have a Witch-King breaking the gate and scaring away the defenders. There do seem to be various breeds of 'elite' and lowly grunt orcs, but the thing here is that it is necessary in this movie to have the orcs winning at one time and then losing at another, because that's how the story goes. It's a matter of making very clear just how desperate the situation is (invincible orcs) or how great the victory of the good guys (TW peasant orcs).
Or something like that.

One more thing, for another interesting military action in Middle-Earth, check out the account of the Battles of the Fords of Isen in Unfinished Tales. Some nice details there, on the various phases of the battles and the tactics employed, as well as the military organisation of the Rohirrim.

If you're still with me after all that, I want to apologise for this far too lengthy off-topic (I think at least) post. I'm just a bit of a LotR fanboy, as you may notice. But it's 2am and I need to finish a paper on Alfred the Great's little Viking problem by Friday the 13th (and I only just now realize the horrible implications of that date... )