Manage collapse, as it will come... especially when you will think you're already on a good way (famous frozen Rhine crossing). Trade buildings at start are most important for me as it will give money. Get agents asap (especially priests will help stabilizing areas with most Catholics - it should be base you want to save from collapse). Leave Britannia, dissolve most of armies. Base your defense on buildings that gives garrisons (sadly in this mod you'll have to manually do most of defensive siege battles; unless something changed since I was playing it lastly; - not for the win tbh, but rather to inflict enemy big enough casualties so he couldn't freely move forward). Manage rebellions by small armies hired just before rebellion start. Keep an eye on loyalty...

- Schedule therapy so that you don't become schizo after your 100th garrison battle.
- I advise you to reload until the Gaetuli (maurish faction) gets a leader with "admires strong empires" trait. This happens often, and it will provide both an reliable ally as well as a trading partner that will give you gems for your gold mines in Aquitania & Gallaecia.
- At the start of the game immediately disband all your armies & raise taxes. PO is not a concern because garrisons are pretty strong but income is your main issue.
- Pay everyone you can to join wars against your enemies so as to distract them. It works surprisingly well.
- Over the following turns downgrade all your settlements to tier 1
- I advise you to convert & go pagan. Yes the buildings are more expensive but they're more useful for income at tier 4, and most importantly you can build slave markets and wine markets to buff you instead of debuffing you.
- Destroy all the buildings in Illyria & Pannonia. Although you can tryhard to keep them, I also advise you to do the same with the two provinces in Britannia as well as Belgium. Keep the others, even Tripolitana far east in Africa because it has dye and $$. But placate the garamantes.
- Send Honorius on a heroic one-man expedition somewhere hostile.

Then it's just a matter of maximizing income until your first usurper event around 405 AD which will make you "lose" if you already haven't Belgium and the whole of Britannia. This event marks the latest point to start building up your armies. You can do it sooner but not later. Up until that point beeline military technologies.

In your provinces maximize income first then PO. Focus at first on Aquitania, Tripolitana (if you can defend it) & Gallaecia. Build some research buildings but don't build anything cultural, it's garbage. Industrial and commercial are very strong for income.

As Rome tier 2 units and upward are very strong and balanced. I advise you to not use missiles at all bc too much friendly fire & a hassle to move around. Just get some spears, a ♥♥♥♥♥♥♥♥ of sword infantry the heaviest possible, and heavy cavalry to dominate enemy cavalry and missiles.

Originally posted by Bellator:

Base your defense on buildings that gives garrisons (sadly in this mod you'll have to manually do most of defensive siege battles;

That is simply not truth, if you manage well with supporting armies to be around most can be resolved with autoresolver.