AU mod introduced more expensive artillery pieces: catapults are 30 (were 20), trebuchers 35 (were 30); whereas, AI is known to be not using them offensively. Spending 30 shields on catapult that sits in a city waited to be attacked to possibly take out 1 HP IS wasteful. It is just a prize for human player when he captures the city.
I suggest to set build priorities for all AI's (including Koreans) to 'Never' in regards to artillery.