Hopefully making things clearer.
Defensive units defend when being attacked, and do not add to an attacking unit's actual stat defense.
Example, a space occupied with a catapult, and archer. If this space is attacked by an enemy, the archer will defend till destroyed. If your catapult attacks a unit/city, the archer adds nothing to the odds of that outcome.
They are separate units. The only time units can affect another unit while in the same space is if there is a great general, or stated bonus.
In the case a few replies above, "Send an army". When attacking cities, armies are the way to go. In the early 3000's to 2400's maybe a single horse or warrior will make it. But later, it's better to send a catapult army then a single unit.