Cool project and really a very impressive 3D engine, that runs good on a stock C64. I wouldn't have thought, that was technically possible before, in this quality. Also nice, that it works on SD2IEC too.
Great work. And I have a suggestion for an improvement of the control in the game. The C64 can also support a second firebutton (Pot-X) and some joysticks, like for example the "Cheetah Annihilator" or the "ArcadeR" have such a Fire-2 button and also some joystick-adapters (GenAssister for example) have it and in well known C64 emulators, you have Fire-2 anyway. A very nice thing for the control in the game would be, when the strafing (left/right) of the player, could be enabled, by holding down this second firebutton, while moving the joystick in the left or right direction. And when the player let out this firebutton and move the controller left/right, then he should turn around, in the game.
That way, the player would have almost everything, needed to navigate through the levels in a comfortable way, right on a joystick. He can run through the levels, turn around directly with the joystick and when he got involved in a fight, he can start strafing and shooting, by using the two firebuttons. And in C64 emulators, for instance, the user can map this Fire-2 firebutton, to any button on the PC joystick/gamepad (because all the best C64 emulators support this second firebutton), making it perfect for that as well.
And one more small suggestion. In the first level, the color of the soldiers' uniforms, often is the same color (light-brown/orange), as the walls, making it often difficult, to spot the soldiers, when they in front of a wall, or causing them, to be seen only very late, when they already very close. Perhaps another color, for example a different shade of gray, could be used here, for the uniforms, instead of this light-brown? That would make the soldiers much easier to distinguish from the walls and would guarantee a better playing quality in this first level.