'Watchmen' in the Bible

Thy watchmen shall lift up the voice; with the voice together shall they sing: for they shall see eye to eye, when the LORD shall bring again Zion.

His watchmen are blind: they are all ignorant, they are all dumb dogs, they cannot bark; sleeping, lying down, loving to slumber.

I have set watchmen upon thy walls, O Jerusalem, which shall never hold their peace day nor night: ye that make mention of the LORD, keep not silence,