Cannot build wall on a city that was a former metropolis
After a metropolis is lost to a player getting the 5th city improvement of a category, the city that lost the metropolis-addon cannot have a wall built on it.
Helpful to reproduce this bug:
- My other two walls were already used. The wall that couldn't be used is the third one.
- The last remaining wall in the inventory cannot even be selected.
Please fix. Thanks.