[Morthoron]

I rather think the Ring fortified the existing structure, but it might be that Sauron could also tap into the existing and eternal corruption of the earth (see Morgoth's Ring) even prior to the Ring being forged. The Ring merely reinforced the corruption of Morgoth (and Sauron, of any being in Arda, would certainly understand his master's concepts, having completed Angband while Morgoth was captive of the Valar).

If you look at another existing structure, Minas Morgul, built by Gondor, it was already a stout fortification. But once inhabited by the WitchKing (who evidently channeled Sauron's power), it became such a place of dread that the evil power which enveloped it superseded the mere stone of the structure. An army would quail in fear simply coming near to it. How then could an army attempt to storm it?

In another fortification, Dol Guldur, Galadriel laid bare the pits and destroyed its foundations. She did this AFTER the One Ring was destroyed. Obviously, there was enough residual evil exuding from the place to warrant its complete destruction before the land could become wholesome again.