This app is based on "Tenfold" (in katakana notation), a web app that I created in 2010 and published on the Japanese SNS "mixi". This one has been abandoned for a long time now, but it is still available in case you are interested.
At the time, I was doing freelance DTP work, but due to a sharp decrease in work, I was looking to switch to the web work. One of the web apps I created to show off my skills was the mixi game version of Tenfold. The situation is similar to now.
In the mixi game version, the number of squares is 36 (6×6). It hit me that 6 × 6 = 9 × 4, and the idea for this game was born. I thought it was very beautiful, with all the numbers from 1 to 9, and the symmetry.
However, after a while, I realized that this game had a fatal flaw. In the mixi game version, the rule was that attacks were alternated like in Reversi (Othello), but that way the second attacker always had a way to win. Therefore, in order to solve this drawback, I have hastily established a rule that only one move per game, and one of them can discard (pass) the piece. This eliminated the drawback for the time being, but the beauty of symmetry was lost.
In addition, the mixi game version works by linking Flash (ActionScript 3.0) and JavaScript. The decline of Flash also kept me away from this game for a long time.
After all, it didn't really help my skill appeal, but it did get me to do a little web work. I started with a website update job like an extension of office work, and only in recent years have I been able to call myself a coder. Still, it is not easy to reach the point where I can call myself an engineer.
Meanwhile, I lost my job again, so I decided to do something about it, and thought about this game for the first time in a long time. So I came up with a rule that attacks do not always alternate, and the prospect of a simpler, more sophisticated game with a 4×4 squares made it viable. Also, smartphone apps that were previously out of reach could now be developed using only JavaScript skills. The mixi game version of Tenfold was finally here to be reborn with a new look.