For me, it's the figure in the middle who stands out, and was the first thing I saw in this image. No matter where I look, I'm drawn back to him. The reason I chose this piece is to determine why. I mean, he's in the middle, so that's one reason. The rest of the scene is so chaotic, he's one of the few figures you can rest your eyes on. There's also the use of high contrast to focus your attention on him - in this case alternating from light to dark on each side of his head:
But, all the heads have similar contrast. Maybe it's that he's the only figure going against the tide - his head is the only one turned to the left. Is that what attracts the eyes? Then, let's take a look at just the heads:
What do you think? To me, removing the rest of the image completely changes the weight of it, and it's now easily divided into four different stories. Now, my eyes do read the image like a book from top to bottom. There are the four heads in the back (at the top) rushing in a straight line to battle. There are the pairs of heads just below on the left, talking to each other, the four on the right struggling, and a couple heads in the lower half for balance. The most prominent head now is the helmeted one in the strongest light, just right of center. The man we originally focused on is drowned out, it no longer matters which way his face is turned. You can also see a pair of diagonal lines, by matching the heads together, neither of which focus us on the man in center...
Maybe it's the thrust of his torso, moving against the crowd? Well, let's compare him to the rest:
Well, it's true that he's the only one moving in the opposite direction. But, his torso/stance actually matches several figures to his left. It's most strongly contrasted by the flow of figures to his right. There's a division line in the lean of the soldiers that bisects the piece, with the man in the center at the middle of it. That may help, but I still see a lot of chaos, with the only main rhythms being the three rows of torsos, bobbing along the image.
Here are some stronger devices that I think best answer the question. Take a look at the spears. They make an alleyway through the painting that focuses our attention on what's going on inside. Note how the spearman in the foreground has his back turned to us - this is also on purpose. Imagine how easily he would take our attention if he were facing us, distracting us from the larger story.
Now look at the men around this main figure. They're symmetrically divided. Two above him, one on either side, and a pair of heads on each side of him, all radiating around him like a halo. It's subtle enough I didn't see it at first, but I think it explains a lot. He's also the only regular, geometric shape in a sea of complex shapes.
Apart from this, there are also a number of lines that draw our attention to him. I've drawn the primary ones in red: the line of eyesight from the soldiers on his left, and the line of his breastplate, pointing up to his face. The thrust of the arms and contours around him also help single him out:
All these devices work together to magically draw your eye on him, a man unnamed and insignificant in the original story, but crucial to this moment of it. We see Faramir in his shining armor, wounded, on the right. He's being held up by one struggling soldier, while another shields him from arrows. The soldier we care about is holding his cloak, and shouting orders to get Faramir to safety. Why didn't Donato Giancola make Faramir the focal point? Because right now, it's his fellow soldiers who make the story, and he wants us to identify with them. Think again about you as the spectator - who are you in this story? A fellow soldier, standing slightly above the others? Perhaps you're an archer on a low wall, or a bit of ruins? This scene could be what you glance at as you reach for another arrow.
Look at the soldiers shouting at each other. Others right by them don't even notice, due to the clamor of battle. Donato's planned this composition not just on what will focus our attention, but on what it would sound like to be there. You can hear the rush of battle, the cries and clashes, when you look at this painting.
There's also a strong link to historical works such as the Bayeux Tapestry or Paolo Ucello's Battle of San Romano - of not simply taking you there, but memorializing the event. There's a strong sense of choreography and grace going on, that romantacizes the action. Looking at this, you get the feeling you could be standing in a royal hallway in Minas Tirith, a fictional city in the Lord of the Rings.
Final point. Where are the orcs? We can't see any. We see many fearsome warriors charging with spears, but not where they meet the enemy. This is intentional. Remember, mystery is just as frightening as the ugliest monster, often more so. Without the enemy in the scene, we don't know how near they are, how numerous, nor how frightening. This reflects the confusion of war and adds tension to the scene, as does the one fallen soldier at the bottom, indicating that spot of terrain has been contested many times and isn't secure.