[samrohan]

I reckon Tolkien viewed both Rivendell and Lothlorien as havens of peace surrounded by hostile lands. If you think about it, it is rather irrelevant wether or not the orcs and other creatures could go in. Tolkien DID not allow them entrance, for had 10000 orcs entered Rivendell, there would be no story.

He had to create a myth around both those elven citadels. This being associated to the rings of power or the presence of grey elves is left to all to picture for themselves.
Tolkien himself would probably not have answers for most the posts in this website.
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However, my personal view is that Rivendell was protected not only by the elves that lived there but because of its location.
Had Sauron sent an army to take on Rivendell, his army would have been left isolated and far away from Mordor. Sauron might have been planning for such an assault by populating Moria with Orcs and such creatures (assuming the orcs there belonged to him: A good question in itself...)

As for Rivendell, don't forget on top of the aura of good and the power of the ring and the grey elves, the Caras Galadhon ( wrong spelling sorry, no book handy for I am at work) was in itself a fortress protected by high walls. The company spend a few hours walking around the walls to enter through the main gates. And for all we know Galadriel might have the power to unleash the forest and rivers on her enemys like Elrond could control his river protecting Rivendell.

In hind sight, it would be nice to have Tolkien with us to answer our questions as he is the only one to be able to, being the Creator.