To totally simplify the game: The best empire is the one who can recruit the most amount of the most cost-effective, battle-effective, troops, per turn.

That means you need a huge economic base, which can be built over a large span of time, or conquered in a short span. That means you need a huge amount of recruitment facilities, which can be built over a large span of time, or conquered in a short span, or built while conquering. Cost-effective troops have armour and good stats, but aren't expensive. And you can recruit large amounts of them. If you lose a large amount of troops in a few battles, you need to replenish them at equal or greater strength, quickly. Can a small empire do that?

More troops means more enemies under seige... means more sacked cities... means more money for more troops.... means more captured capitals and citadels... means more territory, more bases, more economic centers.... means more troops and more buildings to recruit them from... means more and better armies conquering territory... means core provinces can be left undefended... means your budget begins to explode... means more armies and more armies and more armies.

Exponential growth is my strategy. It works. It can be modified to fit your play style, because you can always stop expanding, disband troops, and concentrate on your economy.