Elemental Forums
Zooming in and out different buildings are available to see. This breaks at some point, only closely causal event is building a fortress in a city (see the nice wall in the bottom city?)
A reload fixes the issue. Perhaps something in memory or in the algorithm that decides what gets drawn at a particular zoom level having a missing case?
I get this issue as well; without getting even close to Fortresses at times.
Last game it happened very early, and seemed to simply be related to zooming out (undrawing detail) to cloth map, then upon zooming back into the 3D engine it didn't readd the detail.
Often in place of buildings I'd get a smoke stack showing and nothin' else. 
Yeah, I still get this issue as well. If I save and reload, it temporarily solves the problem. However, this issue happens so frequently that I just play with my "ghost cities." Hopefully, the devs are working on a fix for this.
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