A couple of years ago I had a great campaign as the byzantines, there are two especially memorable battles, both against the Spanish.

In the first one, my army led by an ex-prince landed to some montainous Spanish region. The battle took place on the coastal map where there's a bowl-like valley in the middle, surrounded by steep hills. The Spanish had a full stack of good quality troops, and I had my basic Byzantine setup boosted with Steppe Heavy.

They were half up the hill, but still in a weaker position than they could have had. I marched the main part of the army down to the valley, using one of the Steppe Heavies to absorb enemy missiles and try to get them to charge, which was a mistake, as the cavalry was quickly reduced to 1/3 by the enemy missile fire. My cavalry started forming into a long, perfect line on the coastal side, ready to flank the enemy. There was nothing fancy here, the main lines clashed. But I've never managed such perfect cavalry charges as that day.

In the other battle I had to attack an army of the Spanish sitting on a mountain. I had some mercenary spearmen with me, which is why I probably won the battle: My main infantry force (maybe seven units total, Byzantine Infantry and archers) approached the enemy directly from the bottom, while I sent my four spearmen units along with one unit of Varangian Guards on an approach from the left side of their flank. The main infantry stopped, and I used my spears to lock enemy infantry so I could bring my Varangians to the highest point of the mountain. And indeed, the spears locked with an enemy line running parallel to the mountainside, thus they were on an even footing, no high ground bonuses to the enemy. Yet, the spears were starting to waver, but just in time the Varangians arrived on the top of the line, punched through the enemy, and as the Varangians were now on the high ground, they quickly rolled the enemy line, routing most of the enemy army, who fled downhill to the infantry waiting below.