As usual in this game, the faster you can snowball and abuse game mechanics, the better.
Not sure what difficulty you play on, but from Emperor onwards to Deity, the AI generally has a strong early advantage and you cant mindlessly bum rush it anymore.
So you generally want to snowball fast, and the fastest way to do that is to beat down a neighbouring AI very early, as this means you get access to free cities, you have a standing veteran army (dont lose units), and your core empire (self settled cities) is free to expand and play peacefully behind it.
Generally you can win domination in any era, with the "slowest" point being when the enemy gets medieval and renaissance walls up.
In order to mitigate the power of those walls (and the base combat strength of the city which is the backbone of how powerful the walls are), you need to get good at the tactical and strategic part.
Tactically meaning how and where you place your units (when attacking), and what orders you give them.
Usually the best way is to surround a city with enough melee units (or cavalry type units) to force the city to be besieged, and longer heal up.
You should generally not attack with melee unless you are certain that the city goes down, but rather fortity and heal outside the city and let ranged units do their thing (catapult type units).
If you can train a catapult to its 4th promotion, it gets 3 range and can generally shoot unharmed at an enemy city, at which point its just a matter of when that city drops to 0.
Heck, you can even train a catapult on a city state to have a relatively "safe" XP farming avenue, as long as you keep a melee unit or two nearby for safety.
Generally I run with at least 2 catapults (ideally 3-4) so as to speed up the rate at which cities blow up, or even splitting them if I want to hit more cities at the same time in different directions.
Strategically is a different thing, and relates to choices you make before attacking.
Who is weak, who is vulnerable?
If someone has a low city combat strength, that indicates a low military tech level, and an easy city to mow down.
If someone has cities mostly on open terrain rather than hills and forests, you get a ton of space to maneuver and set up a quick attack.
And very importantly, work on combat strength modifiers before and during every war, you want to stack those.
You can get those from cards, abilities, religion, government types etc., and abusing them is key (by having an intuitive feel of how they shift a battle outcome before you engage, so that you are certain that you win without having to find out the hard way).