1. Enter first room, look at the second door on the left and click on the round bit (take the marble)
2) Look at the pillar and press the button, drag the marble on the small cylinder and unlock the first door. Kill the monster by clicking on the symbol that appears while it is leaping at you.
2a) You may look through a hole in the left most door. I have no idea what good that view is for. This is optional.
3) Go into the hallway. Click on the slightly discolored patch on the left wall.
4) Click on the central piece to break it, then on the one up and left of it, then go clockwise around the circumference to break open the hole.
5) Read the scroll (optional. Does anybody know what it says?) Take the candle.
6) Step back and kill the mummy/zombie 
7) Go back and drag the candle to the torch on the wall to light it.
8) You will automatically go to a room with two unlit torches. Light them with the candle (drag it on the left torch).
9) Look at the picture on the wall and use the mouse (click and drag) to wipe it clean. Note the position of the suns.
10) Step back and look at the control panel left of the locker. Activate the switches that correspond to the suns (upper row: middle, lower row: left and right)
11) Open the locker and take the shotgun - no reason to smile: there's no ammo in it.
12) Look at the picture again and click it a few times. (Maybe anywhere or just in the upper left quadrant with the moon that looks like something has fallen out - I don't know). The entire picture will fall and reveal a filled and an unfilled patch.
13) Take the tile from the filled patch and put it in the unfilled patch (just click it), a passage will open.
14) Take the tile again into your inventory.
15) Go down the passage avoiding the blades (click on the arrow, not the death heads)
16) Go forward twice and you are back in the room with the four doors. Step back and take the diamond, replace it with the tile from your inventory.
17) Go back in and down the passage with the blades (there will be none this time) until you are back at the place with the two empty patches in the wall.
18) Put the diamond in the right patch and press the button that appears, the rightmost door will be unlocked.
19) Kill the bat, the beast and the mummy/zombie.
20) Open the unlocked door and take a key(?) and some shells. It's payback time  
21) Click on the key in your inventory to go through the last door. You will be attacked by a three headed monster. Just shoot the central head repeatedly until the yellow bar drops to zero (the head is vulnerable while coming towards you and while the crown lights up, but I just keep hammering it all the time). Whenever a head comes towards you quickly shoot it or it will bite you (and eventually kill you).

Note: If you get killed you will be given two choices in japanese(?). The lower one is a 'continue' function while the upper one is a 'start game from scratch' button.