You can customize your city’s buildings by using Crown Jewels which you can earn by doing the weekly City Raids and The Arena. Spend them to unlock additional art and permanent upgrades on your city buildings.
Only one Crown Jewel boost can be applied to each building. You cannot upgrade your Castle. The Crown Jewels will provide the buildings with bonuses similar to boosts but a smaller, permanent version. You will only have to pay once to gain the bonus forever and only the mayor can spend the Crown Jewels for a city.