The source is open (https://10-5.de/game/c2runtime.js, seems to be built with the framework "Construct" so you'd have to find the actual game logic in there) but it doesn't seem to be open source so don't go copy-paste it willynilly.

It’s great seeing our game engine pop up in posts like this!

Thanks for making it!

In case somebody is wondering: The game was made with Scirra's "Construct 2", which is a great 2D game-maker. You can immediately try it (the current version 3) in the browser on https://www.construct.net/en/make-games/free-trial

I'm a hobbyist, and usually use Construct, Phaser or vanilla JS.

I have put the source-file on https://github.com/niorad/Ninio, you'll need Construct 2 or 3 to open it, though (paid version I think).

Tom, please allow me to say thanks for your work! As front-end-dev, I'm stunned by having C3 in the browser. I would highly enjoy reading about its software-architecture and approaches one day, especially since it's not based on any of the big front-end-frameworks.

You might enjoy Ashley's blog posts, he does dive into some of the design decisions: https://www.construct.net/en/blogs/ashleys-blog-2