About a year ago, Bret Devereaux, a historian at UNC, wrote a six-part analysis of the movieverse's Siege of Gondor/Minas Tirith from the perspective of military history. He devotes a fair amount of space to comparing the movie's version to the book, as well, to the universal detriment of the movie (surprise - not).
The analysis starts here, with links to the six parts at the top of the post.
There's some interesting stuff in there! He spends a chunk of time discussing the morale effects of the Witch-King's siege, and highlights just how well Tolkien used his own WWI experiences to write them (in contrast to the movie, the morale battle in the book entirely clears out the gateway for the Witch-King's entry - except for Gandalf). There's also loads of stuff about catapults, and how to capture Osgiliath (Jackson's orcs actually had a decent plan, though Faramovie's response left a lot to be desired), and right now I'm deep into cavalry tactics and why the movie should have ended with a whole lot of dead horses.
hS