What became of the Mouth of Sauron?

[Boromir88]

SpM I have overlooked probably a key opponent in order to solve this (if it even is solvable), here's the quote tell me what you think...

The Black Gate Opens
....He gave out a great cry, and turned, leaped upon his steed, and with his company galloped madly back to Cirith Gorgor. But as they went his soldiers blew their horns in signal long arranged, and even before they came to the gate Sauron sprang his trap.

First, I believe Cirith Gorgor is right around the Morannon. Secondly look, at "even before they came to the gate," would this mean, before he could retreat back, the host came out, therefor he was stuck at the battle? That is what I'm thinking, what do you think?