Flexibility and Potency are my keywords for battletactics.

At the moment i have about 8 full Byzantine armies with a few reinforcements on 3 frontiers. Each one (as is the Roman way) is standardised:

50% Varangian Guard. (From Constantinople, Greece, Rum, Crete, Nicea...)
30% Light, Medium, Heavy Cavilry. (From Constantinople or Greece).
20% Missile soldiers. (From Bulgaria or Trebizond).

I fight precise, calculated battles.

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The Varangian Guardsmen can kill spear, sword and horse with no problems. Some people say they cost alot. I can support them no problem because i emphasise strongly on economic growth.

Cavilry i have equal amounts of Cataphracts (Heavy), Pronoiai Allagion (Medium), and Byzantine Lancers (Light). The lighter the cavilry, the more flanking it does. The heavier the caviltry, the more it fights.

Missile soldiers. I accept only the best. Usually 1 unit of Pavise Arbalestiers and 1 unit of Trebizond archers in the early game or 1 unit of Bulgarian Brigands later.

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Pick a front, deploy the armies, push through all at once. Hold, reinforce, build. Push again...

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Deal with desert provinces by hiring muslim mercenaries. Deal with the mongols with Pavise missile troops.

There, thats my Byzantine tactic.