There's the Eiffel Tower, the Tower of London, the Leaning Tower of Pisa—if you've got a tower in your backyard, you've probably got something interesting going on. Towers are signs of wealth, power, and influence, both in our world and in Middle-earth. After all, to tower over something (or someone) is to be much, much taller than the little people around you. It's proof of power. And considering how much Sauron loves displays of power, it's no surprise that he has a thing for towers, too.
As you might expect, there are towers in The Two Towers. Surprisingly, though, there are lots more than two. And weirdly, Tolkien is never entirely clear on which two he means. He wrote, in a letter to his publisher dated August of 1953, "The Two Towers gets as near as possible to finding a title to cover the wildly divergent Books 3 and 4; and can be left ambiguous" (source, pg. 170). In other words: I don't care, man; call it whatever you want. Let's just get this thing into print.
But later on (after it was way too late to doing anything about it), Tolkien wrote to his publisher to complain that the two towers of the title could be any two towers in the series:
I am not at all happy about the title 'the Two Towers.' It must if there is any real reference in it to Vol II refer to Orthanc and the Tower of Cirith Ungol. But since there is so much made of the basic opposition of the Dark Tower and Minas Tirith, that seems very misleading. (Source, pg. 221)
So Tolkien assumes the Two Towers could be either:
(1) Orthanc (at Isengard) and the orc tower at Cirith Ungol (in Mordor), since those are the two towers that end the two books of the second volume; or
(2) Minas Tirith (the capital of Gondor, where The Return of the King mostly takes place) and Barad-dûr, Sauron's Dark Tower of Mordor.
But Shmoop can think of a couple other possibilities, too:
(3) Isengard and Barad-dûr, since we discover that Saruman's fortress at Isengard is only a pale imitation of Sauron's much stronger Dark Tower in Mordor; or
(4) Minas Tirith and Minas Morgul (which is where the Nine Nazgûl now live). Minas Morgul used to be called Minas Ithil, the beautiful Tower of the Moon. These two make sense as a pair because the towers of Minas Ithil and Minas Tirith (once called Minas Anor) have served as capitals of Gondor at various points in the history of that kingdom. Minas Tirith is still the capital of Gondor, while Minas Morgul has become its twisted reflection. They face one another across Ithilien and the fortress of Osgiliath.
(Shmoopers, if your heads are spinning with all these place names, consider keeping this map open in front of you at all times. It'll help. We promise.)
Let's just recap: there are a lot of towers in The Lord of the Rings, and they can all be paired in various ways that are thematically significant. After all, it's that very ambiguity about the name that makes The Two Towers sound intriguing to the reader, even if it's not immediately clear exactly what these towers have to do with each other, or which towers they are in the first place. All we can be sure of is that most of the major battles—and all of the main centers of power—in Middle-earth happen to be in giant towers. And some of those towers appear in The Two Towers, the middle book.