Of course, some changes were inevitable. It made sense for Caspian to blow Queen Susan’s horn early, rather than well into the siege at Aslan’s Howe, for example. Putting half the story in a flashback, as Lewis did (yes, yes, I know he had Homer and Virgil to justify him) probably wouldn’t work well on the big screen.
But they left out many of my favourite scenes! They ditched the numinous and the light of touch, in favour of catapults and hoplites! The children realised far too quickly that they were in Cair Paravel, for a start. The growing mystery, the slow understanding, was concertinaed (sp?) into the space of a few minutes. Susan never got to show off her archery skills against Trumpkin. And Trumpkin himself wholly lacked the gruff good humour of the character in the book, being merely sour and sarcastic. Oddly, in fact, Nik-a-Brik was far more attractive. The whole business about who gets to see Aslan and when as they make their way along the River Rush was truncated to the point of being pointless and confusing. And, while I can see why the filmmakers might not be quite as interested as I am in CSL’s attempt to harmonize Christianity and pagan myth a la Jean Seznec, omitting Bacchus and Silenus and their followers made the moving trees at the final battle seem like Ent knock-offs rather than dryads, and the appearance of the river god a mere Arwenesque deus ex – er, river. In fact, as in LWW, it was hard not to see the shade of Jackson’s LOTR hovering over this film at times. The siege of Aslan’s Howe, in particular, evoked Helm’s Deep to a degree just this side embarrassment, and Edmund’s trick of falling backwards off a tower only to land on a griffin was clearly learned at Gandalf’s knee.
On the other hand, Prince Caspian was channelling Inigo Montoya at times.
Then there was the stuff they added. Most of it I was fine with. Susan getting a bit sick of being jerked between worlds without notice for indefinite periods, for example, didn’t run counter to the person we see in the book, and it’s good preparation for what’s to come. It's quite understandable, too. The main addition was the attempt to storm Miraz’s castle – King Peter’s Dieppe. It took up too much of the film, and bent it out of shape, rather, but at least in acknowledging that war isn’t unremittingly heroic it had the potential to complicate the story interestingly. That aspect wasn’t really carried through, though, and by the time Caspian and Peter stopped bickering those wasted lives had been pretty much forgotten.
The effects were very good, particularly the segue from “The Strand” underground station to the strand around Cair Paravel as an express train thunders past. (Of course, we must forget that the London underground has no express trains and no station called “The Strand”. [ETA: But it did have a station called "Strand" in the 1940s, it turns out! My bad.] But I can forgive a lot for a good segue.) The White-Witch-summoning was very effective too. Less so was Edmund’s ironic remark to Peter afterwards (“Don’t tell me, you had it sorted”), which seems unlikely from the lips of a 1940s English schoolboy.
But oh - the lighting arrangements at Aslan’s Howe, which involved setting fire to a thirty-yard long trough of burning oil (?), won’t be winning the Narnians any energy conservation awards. It’s probably not the best choice anyway for an enclosed underground cavern filled with people expecting to sit out a months-long siege, but think of the bills! And where was the ‘Off’ switch?