The fall of the Tower of Cirith Ungol

[Nazgûl-king]

But as with Narchost and Carchost, the Towers of the Teeth, so here too the vigilance had failed, and treachery had yielded up the Tower to the Lord of the Ringwraiths, and now for long years it had been held by evil things. - From Return of the King, pg 215

So what do you think was the treachery that made the tower fall into the hands of the Ringwraiths, I have come up with a theory about what I thought the treachery might have involved. Please tell me what you think!

Well, I thought that perhaps a Black Númenórean, the Mouth of Sauron perhaps, might have been involved with the fall of the tower. As a Black Númenórean could he have pretended to be a regular Númenórean to infiltrate the tower and take over it? Maybe he got enough power to have the Two Watchers posted at the front gate, and maybe the Watchers, being able to keep enemies out, could perhaps have kept those in the tower trapped within. Then one of the Ringwraiths could have interrogated the people in the tower and then finished them off.

This is the first time I have tried to come up with a theory like this, so it’s probably not that good, but I thought I'd post it anyway.

P.S. Sorry if there is already a topic, or if this is posted in the wrong section.