There are some cosmetic bugs in this. Especially in the intro,  you need to wait for the vblank before updating screen or colour ram. Ingame the rising water raster is not stable on PAL and flickers in various locations,  probably due to jitter. The worst bug i saw so far is on the 3rd screen,  when you try to jump from left to right to the platform with the mummy. You fall down, but never reach the bottom and die. Essentially the game freezer.  You cmp  $d012 against #$fe that never happens.