The game is made in GameMaker (previously called GameMaker Studio 2). It uses it's own programming language, but it is comparable with C#
I've used Medibang Paint to make the sprites. And I've used TileSetter to make it easier to create the Tile spritesheets.
The sound effects originated from Freesound.org (A direct link to all sounds used should be in the readme.txt). And the music is either commissioned or downloaded from royality free music.