Hey ducklin thank you for playing, we are glad you like the visuals!
To make the light, we took a hard look at Godot's isometric light and shadow example and tried to figure out how it worked. We didn't quite figured it out completely in the end, but could manage to use the bits we understood and make a simple version that fit our purpose. As the game has a very low resolution we could get away with very hacky tricks and a low shadow resolution.
Basically it is just Godot 2D Light with default shadows and a custom shader and texture maps just for the walls that prevents them from getting lit from "behind".
We hope that explanation helped! :D