... Not only did the AI keep all 13 Catapults in the city where I placed them ...
What might happen if you placed those catapults OUTSIDE a city??
Answering my own question: I tried a DEBUG with them outside, in 4 different adjacent tiles. When I captured one stack of them with a tank, the rest (finally) fled -- into the most endangered city, as it turns out -- 5 or 6 artillery and 3 or 4 ground combat units (2 of them conscripts).
IOW, nerfing artillery units
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