Add or Move the upgrade building button to Building menu
I have suggestion that I think will be helpful for less clicks and faster way to upgrade buildings, if we have upgrade buttons inside building as well.
So you select that "upgrade" tool, and then you click on the building that you want to upgrade, when you mouse over you have to see the resources required to level up and time to finish, and when you click it must show pop up ask you to confirm and shows the resources that needed and time to finish.
It will be helpful for level up the mines in faster way without select mine and lose the focus.