Good job. Just for the clarification, it is well a voxel engine, but not a "2D" one as it makes no sense (voxel means VOlume piXEL, which is a 3D unit box, compared to a 2d unit-rectangle for the pixel), but a real 3d voxel engine. The good old method to make a voxel like that (specifically, a terrain) is to iterate over the screen column then to iterate over the 'z' coordinate in (3d) world-space. Those loops, for screen space, for world space, is quite an interesting hack of early graphics programming. However, in the case of this game, it seems the renderer works more like a for x-world for z-world, draw square.