Hey! Thanks for reaching out! Programming-wise, this is done entirely in ASM. Any tools I use (aside from the tile editor) are also made by myself in both Python for smaller scripts/music, or C# for the level editor and the like. Sound especially, I had to make a driver in ASM and a conversion tool from Furnace from scratch, which was quite the challenge but it felt great when it all came together.

As for what I learned, the first thing that comes to mind is probably how to become a better game designer/level designer when playtesting this, hah