The AI is very aggressive if it thinks you're weak. You can't have one garrison unit per city and dump everything into infrastructure like in Civ; you'll get stomped on.
Highly recommend this at $20, much less $7.50. DLC is worth it just because there are some fixes/improvements that didn't get backported to the base game; I don't actually care for the new faction/espionage/aquatic nonsense all that much.
This thread on the Steam forums is pretty interesting reading.