Normal Map Lighting is a layer post effect that treats the RGB colors in a layer as normal vectors and lights them like a 3D shape! Best used with isometric art, you define the direction each surface faces by painting red, green, and blue walls, set the light direction in the layer properties, and the shader does the rest.
Paint right-facing walls red (255,0,0), left-facing ones blue (0,0,255), floor planes as green (0,255,0), and any ramp a combination of those colors, e.g. a right-up ramp should be yellow (255,255,0). Set the blending mode of your normal map layer to “Multiply” to have the lighting “shade” layers below it.