With today's update (July 11, 2022), a new feature has been added to the Cities - Monuments.
Monuments are created by combining several buildings into one new building. This feature was designed to help players who have an excess amount of buildings in Storage and wish to condense their collection.
A Monument will collect the same amount of Coins as the buildings that were combined to make it. To get started, just click on the Monument Maker button in the City. This button will only appear in a City if a player has enough buildings to make a level one Monument.
Once a Monument has been made, they can be upgraded to the next level by combining more buildings. Only buildings of a specific tier can be used for each level of a Monument, so if you don't see some of your buildings listed to combine, this is normal.
When creating a Monument, there is a chance to get a rare Monument. This Monument will still function the same way as a normal Monument, but it will just have a different appearance. If the rare Monument is upgraded to the next level, the new Monument will also have the rare appearance.