I played a match against 3 hard AI on mountain pass and walled up the bottleneck with stone walls. When the walls were protected by keeps and archers, the AI attacked with their armies as expected. However, once I added trebuchets behind the walls the AI stopped attacking entirely. I found this strange since I had built a wonder and expected the AI attacks to become more powerful. Upon checking the replay, I saw that there was a moment where one of the AI armies went to attack my walls but received fire from my trebuchets and retreated. After that, all the AI factions continued gathering resources and amassing armies but they would not leave their bases and ultimately did not contest my wonder.
Hard AI stops attacking when bottleneck is protected by trebuchets
Thanks @eeporgorg402. Specific examples like this one are super helpful for the AI team. Appreciate it!