Towers were tall, usually slender man-made structures used as lookout points. In the world of Arda, named towers were prominent, symbolic, and sometimes held great character.
Before the Two Trees
Whilst dwelling on the island of Almaren, the two most prominent towers in all of Arda were Helcar and Ringil which held the great lamps of Illuin and Ormal that lit the world in its youth. They both were thrown down and destroyed by Melkor, causing a great change to the world.
Before the Sun and Moon
When the Noldor returned to Middle-earth in pursuit of Morgoth, many towers were built for the observation of the enemy in the North. The tallest and most prominent being Minas Tirth built by Finrod to watch the northern plain of Ard-galen, until it was conquered by Sauron.
As the Beleriand and its great civilizations were destroyed at the end of the First Age, the only great towers in Arda were located in Eldamar and Númenor. The elves of Tol Eressëa built a great tower in the eastern part of the island used for observing the sea. Men of Numenor yearning for the west could see its lights from both the haven of Andúnië and from the decks of their great ships.
After Numenor's destruction and the removal of Aman from the circles of the world, the surviving Dúnedain of the Realms in Exile built towers of their own most of which held a Palantír, which they used to order their kingdoms and to communicate with each other during times of strife. In the northwest lied the white towers of Arnor. Elostirion, built by Gil-galad in honor of Elendil was its tallest and most prominent. Northeast of it was the tower of Annúminas in the capital, and further to the southeast was the tower of Amon Sûl on Weathertop.
In far southeast of Amon Sûl was the tower Orthanc in Isengard or Angrenost which in those days belonged exclusively to Gondor. Then, there were the great towers of Gondor proper in Minas Anor, Osgiliath, and Minas Ithil all of which held one of the seeing stones in those days.
In the turbulent times of the Third Age, many of the great towers of the Dúnedain were destroyed, abandoned or occupied by the enemy. The most infamous were Orthanc, the Tower of Cirith Ungol, and the Towers of the Teeth, some of which were originally built by the Dúnedain after the war of the Last Alliance to keep watch on Mordor, but were abandoned due to the waning of the kingdoms of men and the Great Plague. Orthanc was occupied by Saruman and soon became a place of evil rivaling Sauron. Only the Tower of Ecthelion remained good and was seen as a symbol of hope.
After the War of the Ring and the restoration of the great kingdoms of men and that of peace, the great towers of the enemy fell and were destroyed. Orthanc still stood and was for a time ruled by Treebeard until he gave it up. Barad-dûr, the ultimate stronghold of the Dark Lord Sauron, had been the highest tower in all Middle-earth during this age.
Fourth Age and beyond
Orthanc remained into King Elessar's time, and was found to contain many of his ancestor's heirlooms and treasures hoarded up and hidden by Saruman. It is unknown whether new towers were erected by the men of the Reunited Kingdom.
"The Two Towers" could refer to any two of the following towers: