Science Fiction : search and destroy

I have just finally read Christopher Priest's Inverted World, discussed at some length earlier in this thread, and I really liked it but I have questions. SPOILERS TO FOLLOW: Can anyone who's read it weigh in on to what degree the physical effects of the "inverted world" described by Hellward (what a name) are "real" and to what degree they're the product of a consciousness warped by the effects of the weird energy field? Because there is some reference to the natives talking about "giants," which would suggest that the energy field has physical effects that are perceptible to people outside it as well as those inside it. But that doesn't quite seem to gibe with the ending, which more or less suggests that the effects are mostly on the perceptions of the people within the city, rather than on the physical world itself. Maybe it doesn't matter because Priest's main points are allegorical, but it did leave me wondering.