That’s quite late in the game. For earlier conquest, use bombard units. They will take down medieval and renaissance walls. Positioning them to fire on the city is a pain as they usually cannot move and fire in the same turn. Try to get the Expert Crew promotion which allows for this ability. Use a Great General to give your bombards one extra movement point. They can fire in the same turn if they have 2 movement points left.
Later on, once balloons become available, build them right away. It extends the range of siege units by one which allows you to take out cities without exposing the siege unit to defensive fire from the city.