The new upgrade feature is in development and will be shown in the new version 0.4 in three weeks.
What includes the upgrade feature?
The upgrade feature brings a new building in the game: The smith. After you have built the barrack you have the ability to build the smithy. Now you can choose to upgrade your defense or your attack. It´s not very cheap, but hey ? This feature brings also a complete individualism for your own city, because now every faction have their own strenghts and weaknesses, one is better in defense, one better in attack. And this was only the first phase. In the second phase (hopefully included in 4.0) you can upgrade your buildings and workers for a higher productivity. So I hope this teaser shows you one small thing for 4.0 and you enjoyed that.