You are correct. In my limited experience, it seems the AI does much of its city defense by attempting to establish zones of control around their city. Often, you can knock out one of these outer guardians, then just waltz right up to a poorly guarded city. Another option is that they often can be outflanked.
Also, they tend to build forts for this purpose, but then leave them empty or poorly manned, providing wonderful, protected launch points for my own attack. Otherwise, you can just use dips to get past them.
I'm still new at this, so maybe someone else can say if the AI still uses these (poor) tactics at Emperor or Deity?
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There are some who call me...Tim