Where It All Goes Down
Middle-earth: Rohan, the Forest of Fangorn, Isengard, the Dead Marshes, Ithilien, Cirith Ungol, Mordor
The Two Towers is a good title for this novel, and not just because it includes two towers in it (two at the least; for more on the many towers in this book, check out "What's Up With the Title?"). In fact, nearly every setting in this novel acts as a point of comparison for another place in Middle-earth.
For example, Rohan is the first major human settlement we see in these novels, and it stands in sharp contrast to Gondor, which represents the height of human civilization in Middle-earth. Tolkien's descriptions of Rohan focus on its wide-open plains and empty spaces, while Gondor means pretty much one thing: the capital city of Minas Tirith. So the difference between Rohan and Gondor isn't just one of age (since Gondor is a much older land than Rohan); it is also one of country vs. city. We only get to see Minas Tirith in The Return of the King. But our knowledge that Gondor is even better than Rohan only increases our suspense and excitement going into the next book. Plus, it convinces us that Gondor must definitely be a place worth fighting for.
Similarly, the Forest of Fangorn reminds us of the Old Forest in Book One of The Fellowship of the Ring. Indeed, the two forests used to be one massive collection of trees, before people came along and started clearing space to build villages and towns. So Merry and Pippin's battle with Old Man Willow in the first book turn out to be a kind of foreshadowing of the fighting Huorns of The Two Towers. But in the first book, Merry and Pippin are on the bad side of the trees of the Old Forest. Now, they've won the friendship of the Ents of Fangorn. The shift from the Old Forest to the Forest of Fangorn also provides a yardstick by which we can measure the character growth of Merry and Pippin: from tree victims in the first book to tree friends in the second.
Of course, the most obvious parallel between places in The Two Towers is between Isengard and Barad-dûr, the fortress of Sauron in Mordor. Saruman (unconsciously) builds Isengard as a poor-man's Barad-dûr, with its giant engines and industries and its huge central tower. The fall of Isengard foreshadows what we hope is coming in The Return of the King. All that remains of Saruman's stronghold is Orthanc, the tower that actually dates back to the Men of Númenor. Otherwise, Saruman's works get totally destroyed, which might mean bad luck for Sauron in the final book of the series, and also might signify that humans will be the last ones standing.