Barad-dûr was built by Sauron, the Dark Lord of Mordor, with the power of the One Ring, during the Second Age. The building took six hundred years to complete; it was the greatest fortress ever built since the Fall of Angband, and much of Sauron’s personal power went into it.
Barad-dûr was besieged for seven years by the Last Alliance of Elves and Men and was levelled after Sauron’s defeat at the end of the Second Age, but because it was created using the power of the One Ring, its foundations could not be destroyed completely unless the Ring itself was destroyed. Isildur cut the Ring from Sauron’s hand but refused to destroy it, so the Tower was quickly rebuilt when Sauron returned to Mordor thousands of years later, in T.A. 2951.
The Dark Tower was described as existing on a massive scale so large it was almost surreal, although Tolkien does not provide much detail beyond its size and immense strength. Since it had a “topmost tower” it presumably had multiple towers. It is otherwise described as dark and surrounded in shadow, so that it could not be clearly seen.
“..rising black, blacker and darker than the vast shades amid which it stood, the cruel pinnacles and iron crown of the topmost tower of Barad-dûr…”