I dont think Sauron made the mountains, mostly based on the fact that it would take so much power to do it. And I think that such an event would have been recorded in the books. It was, I think, just a very convenient place to build at.
And another reason I think he didn't make any mountains is because it talks about how he had orcs construct the 2 hills infront of the black gate. If Sauron wanted 2 hills he could have made them himself, but it would have taken too much of his power. Now imagine raising a huge line of mountains.
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Son of Isildur.
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