I wrote a game engine to learn more about how they work and it ended up evolving into "Portal meets Doom" iOS game (https://apps.apple.com/us/app/gate-escape/id1449377239)

I've talked a little about the technical details, but not much of the game itself. It makes about $40/month - not exactly motivating money. It's been very difficult to market so I have lost most motivation to acquire more users.

I dream to open source the level editor and allow community made maps, but I can't really judge the demand for such a large feature investment.