what were The Silent Watchers?

[Rumil]

If I remember, the orcs commented that getting past the watchers was 'Tarks'work' ie. Numenoreans' work. This suggests that the Watchers were 'made' by the Numenoreans and had been at the tower since it was constructed.

However, I find it hard to square this with their obviously malevolant nature and with the fact that the orcs had been using this entrance for hundreds of years. The only explanation I can come up with is that Sauron, (or one of the more Sorcerous Nazgul?) broke the 'spell' (or whatever?!) but they still 'remembered' that they should allow Numenoreans in.

Anyone else for speculation???