I am the author of HyperRogue and Hydra Slayer, roguelikes based on mathematical concepts, and several other projects. I believe that art should be available for everyone, but the authors should be rewarded for their hard work. HyperRogue started as a very simple project; then, it got accepted to Steam, where it turned out that people are willing to pay for it, which allowed me to greatly evolve both the free and the paid version. I try to combine the benefits of open-source and paid development: new features are added to the game (at no extra price), and then they are added to the free versions too, possibly after some time (except the some special features which remain in the paid version only). If you would like to support this style, please consider becoming a sponsor!