in almost all AI strategies, the AIs build the minimum garrisons then start working on wonders and or buildings. Thus they dont have much defense against the barbarians. if you didnt increase the garrisons in the strategies.txt, then most of those AI cities will only have 1 or 2 defenders.
Take a look in the AI forum under the thread I started about "AI without SLIC" and you'll get some more answers as we are discussing this very topic.
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