Legendary Eastern Roman Empire Tips

-The first thing's i advice you to do is to give up the european side of your empire (Greek Makedonia and Thracian) or at least the town with no walls in the north.
Keeping Constantinople and anatolia you should be able to defend your west front with only two small armys.
I've keep greek for historical and weepy reason, but it was a bad idea, for some meager income i have to spend a lot to protect it, and wasn't able to avoid raiding there.

-You will start at war with some migrating tribes, try to do all battle manualy, even the hopeless one, with your garison's cavalry you can always catch some onagre and archers, making the next battle easier.

-You start being at war against Huns, they usually don't rush to your empire, starting by making a mess in german's territory. Here you have a big choice, or you ask for peace (by putting some money on "the altar of friendship") with Huns and you will probably be at war with all new german kingdoms, or you try to fight them, doing so you can hope for a good relationship with europe.
Be aware that Huns have a scripted system, you cannot beat them until Attila is dead, they will always come back.
That's the main reason why i've choose peace with Huns but at the price of an eternal war against german's kingdom.

-Western Roman empire, that faction is the worst friend ever, as most of the barbarian will attack them you want to break your alliance with them as soon as possible, hey you have to survive until the XVs you cannot dive with you western brother that early.
You may even want to start a war against them when they collapse , not to take any territory but just to improve your relationship with almost everybody.

-You will have rebels. Whatever you do. But they're is some way to make it easyer to deal with them:

-It can be tempting to improve your public order where it is really bad, but it's actually a bad idea. Your goal will be to make the rebelion arrive at differents moments so you have the time to run from a province to another one with soldiers to "calm them down".
To do so you want to improve the public order where it is almost positive in order to make it really positive and have some "safe places"
To improve public order the first thing to do is to have enough food everywhere, the second is to build an administrativ building in the capital of each province. It's a really cost effective building giving you boosted garrison (and that's useful) public order, money and tax rate, what else could you ask for?
You want to have this building in every province capital, if you have no place just destroy an existing building. The only other building that have public order are the church who cost an insane amount of money and some theater that give some old religion.
About religion the church are really expensive i advice you to use the technology until plumbing for this, trying to go back to paganism might be a solution too.
-You're own moral: We're used to expend from a small province to a great empire in Rome 2 but here you will usualy loose some territory, you will feel like it's hopeless to have a good public order everywhere, but don't despair, even if you use the scorch earth, loose territories, you will at the end have much more province as if you start with a single province. If the game was starting with good public order and diplomatic relationship you would win in a few years.

After a few years most of your territories will have a good public order and most of the migratory folks won't be a threat any more, but three more threat are here:
-On the East the sassanide empire will get over the white Huns at one moment or another, if you're lucky they will have some more inter-war with they're own client-state, allies or arabic folks, but one day or an other they will come for you.
(on progress)

Buildings:

Military buildings are really expensive to run, it go up to 1000 for the last tier and the're is three branches so you cannot afford them all, fortunatly you just need one:
-ranged weapon military building level 3.

That's really all you need, let me explain myself.
Mele:
The legio-comitatense-or whatever technology level you are can be recuite in any military building and they're realy good enough for your frontline. The infantry recuitement building give you slightly better units but with 2 turn recuitement and no javelin (you can hope to kill at least 50 guys with your javelines when on barricade) It's really no worth it.
The spearman you can recuite everywhere is all you need against cav.
Ranged units:
Crossbow/balistary, that's the best, don't upgrade them with the "elite" they will cost the double to increase they're mele/armor, and that's useless as long as you're on the top of the wall.
They're doing less domage as javeline when you care about the time, but in defence you will hold the streets for a while, you won't be overwhelmed so what you care about is the total domage they will do with they're amunitions and for this crossbow are the best, I've reach 1000 kills for one balist many time, and that was not kills made by slaughtering the runaway ennemys.
Oh and vs infantery ALWAYS switch to heavy shot.
Cav:
You don't need the cav building either, in the ranged weapon one you can recuite mounted archers, they're less good in mele as the light cav but that's still more than enough to kill ennemy's long rang units, plus, in the case where an ennemy cav decide to get yours you can always kill them from distance as you move as fast as they do.
Once your done with this you can catch the ennemy archers while they're mele units are busy to make pogo in the the streets blocked by your own soldiers.
The next step is to shoot on this units in the back, once you've run out of amunition you can charge if they start to crumble.

90% of the battles you will have to do on the battlefield (not auto resolve) will be defensive one, usually you will be offensive only after your ennemy crush himself on your defences, after that you will hunt the runaway and strike hard on there border town with usualy the garrison left.