It's just code compiled in web, no engine :)
Playing locked levels is not a bug, but a feature! Allows me to quickly play a random level on a different computer, and to help very stuck players that want to skip levels. Very people find it on their own, congrats!
2 players is a cool idea, wonder how it would work. Does each have their own timeline? Can one undo the moves of the other? I'll have to think about this...
If you want to try ideas for this style of game but don't know js, I have great news! Check out PuzzleScript: puzzlescript.net. It allows you to make grid-based games without code. For example, a moving block is simply "[Block] -> [> Block]"; a wall for player but not crates is also super easy to do. There are a ton of cool games made with this engine (https://itch.io/games/tag-puzzlescript), and it's what I use for the many games that can be done in it (Tres Undos can't, since it messes with undo itself)
Yeah, i know about puzzle script. But you have only 6 buttons in puzzle script (let's forget about mouse for a second, ok? (i think with mouse that's possible to make, but that will be pain lol) ) : arrows, undo (you can't modify it), and action, so you can make really undo is undo and action is fake undo but... idk how to make that. Also, you know about "puzzlescript plus", yeah?
2 players... i think they just should move at same time and have different hats, that's it.
(that easily allows more that two players)
we probably should try to talk somewhere else. like discord?
At the Hedges of Time did fake undo in puzzlescript, but the code is super complex - thinking about it, idk if Tres Undos would be doable in puzzlescript, even with more buttons & being able to hijack the undo button.
I've just remembered that there's a cleaner codebase for the multi undo system: here is the source code for this game, which is basically Breakout but with white/orange/red bricks. If you were to make Tres Undos with multiple players, it might be easier to remake it from scratch taking this as a base, than trying to modify Tres Undos directly.
My discord tag is knexator#4865, feel free to DM me.