what were The Silent Watchers?

[The Saucepan Man]

From The Tower of Cirith Ungol:

They seemed to be carved out of huge blocks of stone, immovable, and yet they were aware: some dreadful spirit of evil vigilence abode in them.

This suggests that they were, in physical form, statues, but that they were imbued with some kind of spirit. And since the spirits inside them are said to have been evil, it seems likely that they (the spirits) were placed there by Sauron, rather than by the Men of Gondor who built the Tower.

What is not clear is what the spirits within the statues were. Quite possibly, they were not "living" spirits at all, but a form of enchantment - an artificially created and unliving awareness (a sort of ME motion sensor [img]smilies/wink.gif[/img] ). Alternatively they may have been living spirits, possibly of some lesser form of Maiar, removed from their bodies and trapped within the stone. Personally, however, I think that there is a lot to be said for Meoshi's theory that Sauron imbued with them with a small part of his own will, in the same way that he invested a (greater) part of his will in the One Ring.