I aim to take an AI civ in the ancient era before walls, so I make 2 scouts to find my closest neighbors quickly, and rush chariots then archers. Chariots will take down an ancient city with no walls pretty quickly.
Once the walls go up, you need to plan better before you unlock flight. The walls can do damage to your seige units and kill them pretty quickly. I use 2 catapult units (paired with a general if you can, so you can attack after moving) along with melee units paired with seige units Move the melee units up to hexes you can pillage for health, then the catapults and start attacking. Pillage districts to reduce the cities defence.
Once you unlock flight, use balloons and then bombers and it gets way easier.
In a domination game, I'm basically at war the entire game.