Hello. I don't know why you want to delete your Ballista, but if you are sure of it, you can follow my screenshot. Just be sure that the arrow is pointing at your ballista and not at your tower.

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thank you. the only reason I wanted to delete it was because there didn't seem to be a way to upgrade it. I thought you had to highlight the ballista to do anything to it. You might want to consider changing that in the game ( to allow highlighting of or clicking on the ballista, that is).  I have spoken to other players who weren't aware of this either.

  Hello Copperlady. I do hope that you will upgrade your Ballistas now. It is understandable if you thought that the towers and the Ballistas work like the rest of the buildings when we want to upgrade them, but it is like this since we started the game.  Frankly, I don't think that the developers will design a highlight option only for the towers and to move buildings and decors.  Of course, Daria reads regularly what is posted, so she might keep your suggestion in her mind.🙂