I've played 3 campaigns so far, Nicaea, Zengids, and Nicaea again. Currently doing Nicaea 2 with This is Total War, which is war with all factions and no diplomacy. As you can imagine I've fought many battles and have some experience with the Byzantine army. It's strange to see talk about Nicaea as if their cavalry is weaker than "knightly" Western factions: Cataphractoi are supreme in 1212. They are the second strongest cavalry unit in the early era and thus the campaign, only the Ghurid Ghulam-i-Sultan is better. Knightly factions actually have mediocre to somewhat weak cavalry in the campaign, Christian knights aren't that strong in the early era and don't take off till the high era. Cataphractoi have the 190 health - 155 charge - 40 missile block - 55 armor holy grail of stats that none of the Christian knights are able to come close to matching. Their massive health, armor and missile block make them highly resistant to damage especially from the front, their charge allows them to break anything even without wedge, and their very high health, armor and melee defense make them highly resilient even in drawn out melees. Cataphractoi are the ultimate Christian unit and can take down any force or city if they have the right backup, which they will need to make up for their slower speed.
I'd like to see some of your army compositions. Heavy cavalry is only strong if they have friends who are watching their back. Artillery, archers and crossbowmen are good friends to cataphractoi. Try to keep 1 or 2 melee infantry on each of your flanks further up, midway between your main line and cavalry flankers so they can run forward or backward to help either, depending on the situation. If archers are giving you a problem then brings lots of your own. The AI in Attila really does not like its ranged units being shot at by other ranged units, you can easily suppress their archers by shooting them with your own. The AI will relocate their archers most of the time. Even if their archers are better and would beat yours in a straight shootout, they will still relocate when shot at. The Attila AI really,
really does not like its archers being shot at.
My armies are all variations on the same 7 infantry - 5 ranged - 6 heavy cavalry - 1 catapult - 1 general theme. They look like this:
Army 1
Army 2
Army 3
Those are my main armies, each of the 3 also has a stack of 19 catapults following it for easy city sieges. Strategy is usually: use the catapult to force the enemy to attack. Put infantry into shield/spear wall. Catapult focuses down enemy cavalry and general. Archers prioritize shooting at units that were hit by the catapult, their hitpoints will be low due to bombardment and die quickly to focus fire. Use archers to focus enemy ranged units to suppress them from firing. Use Cataphractoi to stop charges on infantry flanks. Circle around with Cataphractoi to knock out general, corral enemy archers, and hit the rear of the enemy line. Once the general dies the enemy army folds quickly.
Here is a replay of a siege I just did. It shows some good work by heavy cavalry and how strong they are at surrounding a larger force and destroying it quickly. Two good things in this battle:
1. The beginning, I rush the enemy reinforcements that are landing on the shore with cavalry that is completely unsupported. A good surround routes them all very quickly with almost no losses.
2. Toward the end, I clear the city out completely using my heavy cavalry. They are well supported by archers, crossbowmen, artillery, and pikemen to make their job easier.
There's a long boring period of sieging the walls between the two but it still may be useful to see.
edit: well I just tried to load the replay and it crashes upon loading. I guess replays are broken in this mod, that's too bad.