This translation, by Mark Harmon, is based on a thoroughly revised and critical text (- a critical text is one in which the manuscripts have been reexamined, variants weighed on philological and scientific grounds, and editorial decisions made on a defensible basis -- i.e., not on a "hey, this sounds cool!" foundation.)
The old edition, translated by Muir, was based on Brod's original text, which had done great violence to the original manuscripts by trying to regularize the writing, the language, and by reducing any 'fragmentariness" of the manuscripts (even though all these 'irregularities' contribute to the effect, and much of it was deliberate -- or in the case of the fragmentary nature of the ending, simply factual). Brod was interested, of course, in reaching a general public, and not in accuracy.
The text of the Castle was redone in a critical edition in 1982, and this translation is based on that. Among the major changes -- in addition to differences in the actual text printed, what is included or excluded, is that Harmon reverts to Kafka's original style of punctuation -- which was quite unique -- and which had all been regularized by Brod into High German punctuation rules. Kafka, however, simply strung together long clauses by commas -- so that the run-on of events and thoughts has a certain breathless quality and "open-endedness" to it (and a modern quality) that is obscured in the older translations. These are significant changes, and probably makes this book quite different than the Castle most people know.
I thought I had read this book in my previous life -- I had read a lot of Kafka, I think, as a teenager -- but I guess I probably missed this one -- and so it's a perfect fit for my current 'project' of hurling myself back into (at least) the *recent* past...