Dr. Bret Devereaux

So he beats up the front of Sauron's army and then gets out of Dodge.

And...

We don't get details, but it's not hard to imagine situations where the way the battle develops, he can't retreat back into the city.

Or alternately, he doesn't want to retreat back into the city because he wants to maintain the ability to maneuver and not get locked up.

He's like, he knows the city is going to get locked up.

Like, I want to stay broken out.

There are defenders inside the city.

They'll last for a long time.

I want to stay out in the open where I can maybe get around and cause some mischief.

He loops up with Elrond.

Sauron knows this has happened.

So presumably, like we're told, he has a strong enough host to also block out Elrond.

We're not told, but what I think we should assume he's doing here is contravelation, where now having built a wall facing inward, you form your army into a donut with a second wall facing outward.

One of the really famous examples of this is Julius Caesar's Siege of Alessia, where he's got Vercingetorix's army in this hilltop fortress, and so he builds a Roman defensive line around the base of it, as you do.

Mm-hmm.

The Romans love themselves some field fortifications.

And then because Vercingetorix has gotten messengers out to call for help and he knows a whole bunch more Gauls are going to show up behind him, he builds a complete second line of defenses facing outward to prepare for their arrival.

And they end up trying to break through his forces and failing.

And this leads to Caesar winning as he usually does.

But so you end up with two complete lines of fortifications facing in opposite directions and your army arranged as kind of a donut.