Captured (and thought I destroyed) Samarkand with a size of 1 - window pops up saying I've destroyed the Mongols. Move my unit away from where Samarkand "was" and it's still there, still Brown, and undefended. I can click on the city and view it (as I would be viewing it with a diplomat) and landing back into the city does nothing, in fact my unit blinks on top of the city! Eventually, the dead Mongols manage to build another military unit and I can destroy the city and their civilization a second time. Just another quirk?