Before continuing, I'll answer something.
The Romans have not met Giants, there is no official position on them. Most think they are some sort of myth. the Church has no official position, but once or twice an adventuring priest has wandered beyond the wall, seeking to preach to the Free Folk, Giants and Children... only once has one such priest returned, and he did not meet any Child of the Forest, or any Giant.
Duel of the Dragons
Somewhere in the coast of the Gods' Eye
Both armies slowly formed their ranks. And here it was that Rhaegar first noticed his plans unraveling.
"A Vexing sight indeed" Said Lord Connington "I cannot quite understand why would the Romans set their light infantry in the center..."
"It could be a ruse, their Flanks are still covered by Spearmen, So we can still execute our plan." Replied Rhaegar, both men watching their enemy take shape among the morning mist.
"Aye, we can still crush their flanks" Was all that Jon could say, he soon rode to his men and waited for the bugles.
"Your Excellency, Commanders Marius and Aurelian are here"
Cornelius nodded and signaled them to approach him. the three of them mounted on their horses.
"Something's amiss here." Said Aurelian "Seems to me they are preparing an attack on our flanks first"
Cornelius looked at the enemy forces and nodded at that. Thinking.
"We will retain our current plan, Commander Aurelian, you will command the cavalry on the left wing. You are to repel any charge, but do not pursue. We will need your forces for the envelopment."
Aurelian nodded, Cornelius looked at his men.
"I will take the reserve Cavalry, once we begin surrounding them, we will ride beyond their rear and leave a retreat path open to them... Our objective is to force them to rout," He said calmly, looking back at his commanders "Commander Marius, Can we anchor the Right flank to the coast?"
"I will see it done" he said, soon enough he rode towards his men. Cornelius and Aurelian soon did the same.
"That might be a problem." Bluntly stated Ser Harrold, looking at the Prince. Rhaegar had also noticed the small shift on the Roman formation, anchoring themselves on the lake would make his strategy harder to do, but then again, he could still use the left as a ruse and break their center.
"Ser Harrold! Ser Arthur! We will charge their center once their Left is reinforced!" yelled the Prince
The bugle sounded then.
The Knights charged forwards.
Cornelius had done another last-minute change. He was now with the light infantry.
"Hold the Line!" He yelled. Of course, 'light' infantry being those equipped with a large oval shield, a gladius-like short sword and mail armor instead of scales or plate.
The advantage of this type of infantry, is that it could be trained to fight like their ancestors did before exchanging the gladius for the lancea and the spatha.
They were still vulnerable to a determined charge, but even mounted knights would think twice before charging a shield-and-sword wall.
The Roman bugles also sounded, the infantry closed ranks, and Cornelius waited. Soon he saw the Knights slamming into his cavalry at the left. All he needed to do was to wait.
The day unfolded, as the sun rose high into the sky to signal noon, the larger part of either army still had to see combat, for three hours both armies had been holding back, the Roman infantry in the left was tired, and if it fell, they would be cut down. Then it happened.
One knight, one Ser Gerold, decided that he would charge in and gain some glory. Almost like a stampede, impatient knights followed. They were racing towards the Roman center.
"Face Forward! Retreat!" Cornelius yelled. The Roman lines seemed to bulge backwards in a crescent as the Light infantry marched backwards, creating a bowl of Romans into which poured knights, men-at-arms and smallfolk levies alike.
"Idiot" whispered Rhaegar, as he hurried his knights to the left flank. If he could break them, they would be able to win the day.
The Roman reserve cavalry hurried to the beleaguered left flank, while no one saw the archers, crossbowmen and reserve infantry marching to the shore of the lake.
As Romans and Andals fell alike, another bugle was heard.
When the Knights made contact with the Roman center, the Spearmen on the right began to move.
It took perhaps two hours for the Roman right flank to reach the rear of the Andal warriors. With their rear being cut down by spears and swords, the right flank of the Loyalists became their only way to escape.
Rhaegar also took notice, recognizing what his half-brother was doing.
"Cannae" He whispered, then called for a retreat, before they could be fully encircled.
The Roman left broke, but his men were too busy escaping to take the chance. The knights that were with him were not enough to turn the tide even if they rode into the Roman rear. Or at least so thought Rhaegar.
The knights retreated in good order -mostly- as did the men at arms... the levies routed and were massacred.
At the end of the day, the Romans stood victorious, but too many had died or been wounded to pursue the Prince and his forces.
Two days later, as Cornelius' men were licking their wounds, A Raven arrived.
Cornelius and his remaining men were to move northwest, to meet the main Roman force, which had taken position futher to the south than initially planned.
Rhaegar for the time, managed to retreat south, towards the rest of the Crownlands Levies, and the first few Reachmen forces to reach the area. When inquiring about the rest, he learned that the main Tyrell forces were with Lord Mace Tyrell, laying siege to Storm's End.
Thankfully, the few Reachmen that reached his lines, were enough to more than double his army.