Pressing Attack and watching the skeleton, mummy and ghost units trade hits with the red enemies was easy to read, and reaching Stage 25 with +312/s income and a Stop Battle toggle gave a good sense of progress. The greyed-out locks (lvl 10, lvl 30 and the #38 unit) are a nice teaser, but nothing says what they unlock. A one-line tooltip on each lock would tell players what they are working toward.