All the useful gold-producing buildings can be upgraded. Unfortunately, you can't merge them, but you can use gold to increase their level and make them more profitable! To level up the building, press the green button with a + on it.

 

 

The number on the button indicates the number of levels you can afford with your current gold supply. The upgrading cost is also shown on the button. Using the blue bar above, you can estimate how close you are to the next big income boost. The light blue part of the bar shows the progress you'll get after upgrading. The yellow part shows your current progress.

 

Plan your budget wisely: upgrading the building by a few levels might not yield the increase you want. When you reach an income boost milestone with your building, you get a small number of bonus elixirs.

 

Another way to permanently boost the building's profit is by researching more advanced technologies using cards.

 

 

Don't worry, your elixir wasn't wasted! It means that your mineshaft's card was upgraded, but the new level is not enough for automation. You might need to upgrade the mineshaft's card a few times until you can automate it.