Some buildings have the ability to be upgraded to increase the capacity of their warehouse.
Some buildings allow you to unlock additional slots for new characters of the same type using resources.
The cost of unlocking a slot for a new character is 150 resources of each type
If the building is NFT, then when added to the game, its level will always be 1. During the Wipe, the level of Buildings is also reset to 1.
Some Characters can be upgraded by leveling up to increase the base resource extraction (if that character can extract), but you also increase the token crafting time.