Zillion 3D is a Wolfenstein 3D mod with characters from the anime Akai Koudan Zillion. Two episodes. 20 levels. 4 bosses. It’s based on Wolf4SDL, so it runs straight from Windows.
Here’s the deal: you should not waste time and ammo with anything else besides the main quest for the two keys. Don’t enter rooms that you really don’t need to, and always “peak” inside before entering. Health and ammo are scarce, so take them only if you really need: memorize where you left them so you can backtrack to heal yourself before the last legs of the trip.
The keys are in the uppermost left and right corners of the map inside hidden areas. The first is behind a green wall (1) with the yellow Maris army logo, while the second is behind a red wall (3) at the end of a corridor filled with nozahs.
I usually follow the same path every time, starting with the green key up on the left corner (1). I kill and take only what I really need. With that key, you’ll have access to a room (2) on the opposite lower corner of the map filled with ammo, food and the chain gun. This room is crucial, since you’ll probably need to head back there to heal and reload more than once. With all items maxed, I head to the blue key on the upper right corner (3). You have to pass a red corridor filled with enemies at every corner, so I usually backtrack to room (2) if my health drops too much before I reach the key. Once you find the blue key, it’s time for the last leg of the trip: the two inner circles that house the elevator (4). The green key opens the first circle: there are four flying pao paos in there, so be sure to stock up from room (2) before opening that door. After killing the pao paos, open the last door to the innermost circle with the blue key. Inside there’s eight armored nozahs so, once more, be sure to have ammo and health (usually from room (2), though by this time whatever you can find helps!) Once you kill them all, you can open the elevator door and say good bye to that nightmare of a level!