[djur]

The commercial release of Celeste is a new game, written from scratch, implemented in C# (XNA/FNA/MonoGame). It includes a port of the original. I'm kind of curious if they actually wrote a PICO-8 emulator for that or just ported it to C#.

[barbs]

They have a Github repo with some of the source code up (mostly just stuff to do with character movement and control).

https://github.com/noelfb/celeste

Apparently it's based on this framework:

https://bitbucket.org/MattThorson/monocle-engine/src/default...

[AlyssaRowan]

They ported the PICO-8 version line by line to C#. Since there aren't that many lines, apparently it wasn't that hard.