Attacking this celtic city (the very NW tip of Europe in Armoriae province) and trying to fight the (siege) battle on the battlemap will cause a CTD. Either autoresolve it or leave the city alone. We have figured out the problem (we placed a unique building - standing stones - on that location), and are trying to get a fix for 0.81 in it. But for now, be warned that you can't fight a battle on that city square on the battlemap.
Thanks go to Damasus, Dram, and Swindler for posting on the problem, which led to us investigating it and figuring out what was causing it.
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