Thank you very much, I'm glad you like it. Now you've pointed it out, I agree that 'upgrade' is suboptimal. Maybe 'build attack tile' would fit better. And yes, I understand the information problem. Initially, I was thinking of a more fantasy-style approach, but then I decided to go with a "keep it clean and simple" approach. It's difficult to deviate from that now.
You can just click a tile and then press the 'Upgrade' button in the bottom right corner.