Sorry to hear that. Of course that is some weird bug that is occuring when all the planets come into alignment -- In all of the games we have played, I have never seen that. There is a unit in the game, called the King, that is very hard to kill but then he is the king!
I know its a rotten deal you got, but think about starting another game. I can't imagine what is causing that? Do you know if it had wall around the city? A parapet? That could have caused some kind of weird anomaly.
Its was pretty early in the game actually. But im on my XPS laptop now, hopefully its not there....
Ill get back to you on that. Maybe its some weirdness in the Firaxis folder after tons of Mods.