Am I the only person who finds this extremely difficult against aggressive opponents. On my game (only on Warlord), I'm against Caesar and Montezumo. Completely out of the blue Caesar declares war against me, and before I know it a huge stack of units is taking over my city.
It seems that the computer just batters you until your defence cracks, nothing to do with skill.
How do other people on here defend against aggressive opponents? Does the computer spot poorly defended cities and go for the kill?

Generally computer players attack people who have a lower power rating, which is how your total military force is calculated. If you are significantly lower than the AI they will attack you.

The best defense is to spend more time building units than those coveted wonders. Also, only build buildings that you need and only build them in the most pertinent cities. Consider having at least one city that ONLY builds military units and related buildings.

Apart from taking care that your power rating is not too low, you should consider your strategy once they attack you. Attack his units with your siege weapons and finish some off with other units. Fortified archers and longbowmen are better off fortified in the city, but a number of units do not have any such benefits and are better used to attack the enemy in the field, preferably ending the turn within the city. Mounted units can also destroy or at least seriously wound his siege weapons through flank attack ability.

Am I the only person who finds this extremely difficult against aggressive opponents. On my game (only on Warlord), I'm against Caesar and Montezumo. Completely out of the blue Caesar declares war against me, and before I know it a huge stack of units is taking over my city.
It seems that the computer just batters you until your defence cracks, nothing to do with skill.
How do other people on here defend against aggressive opponents? Does the computer spot poorly defended cities and go for the kill?

Have units on your border as an early warning.
Hit their stack well before it approaches your cities.
Use siege (Catapults, etc.) to decimate the stack.
Keep an eye on what units they're building, and have lots of the counter-unit.
(if you see lots of Axeman, build Chariots)
Yes, the AI will find your worst-defended cities.
They may not attack if you can keep your military near the top in power.