About this creation
The most complete description we have of Barad-dûr a quote in The Two Towers. We know that it was black, and constructed mainly of metal. 'Adamant' usually refers to diamond, but it is hard to imagine Sauron adding diamond towers to his dark fortress - in this context, it probably has the more general meaning of 'hard, unbreakable substance'.
From the steel gate of the Tower, a causeway ran out into the plain of Gorgoroth, across a mighty bridge. We also know that lava from Mount Doom was channeled back across that plain to Barad-dûr.
Tolkien probably leaves details of the Tower's construction vague intentionally, as it was hidden by a cloud of shadow and darkness at all times. The only characters in The Lord of the Rings to actually see Barad-dûr were Frodo and Sam as they journeyed to Mount Doom: '...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...'.
The 'topmost tower of Barad-dûr' seems to have been the place where Sauron dwelt. At least, the Window of the Eye, from which the Lidless Eye stared out across Middle-earth, was located there.
