The final cavern will throw every trick in the book at you, including a few new ones. It starts off with a staircase whose steps disappear as you jump on top of them. To get through this passage safely, keep the jump button held down and continue to push right. You will bounce off each step as it disappears. A few other sections contain disappearing blocks, but don't forget that at full speed, you will run right over gaps, so you don't have to jump over every tiny space. The enemies will come out in tight groups, which will make holding on to your health difficult. It is best to proceed as slowly and as cautiously as possible, so as not to trigger the appearance of an abundance of enemies. To reach the final secret chamber, you will have to go against what the game has taught you and take the low path of the three way split in the middle of the stage. When you about two squares to the left of the block which disappears for the wall, push down to access the elevator to the secret room. Inside, you will find nothing more than the letters LA scribbled on the wall, presumably for Layla.
There are two important things to know about this fortress. The first is that you will have no choice but to navigate all the way through it twice. The reason being in order to get the password disk, you will be forced to take an exit which deposits you back at the start. The password disk is particularly difficult to find, since you must reverse your direction on the elevator and arrive somewhere completely different than you just came from.
The second important thing is to avoid the third and fourth floors, as well as the second and third basements entirely. If you wind up on either one of these floors, you will become forever trapped in a never ending loop that will prevent you from reaching the sections that you need to reach in order to complete this fortress. You must always make an effort to remain on the first and second floors and the first basement, until the time comes for you to access the fourth basement. Failure to do so will result in you having no choice but to die and start over or reset the game and reenter your password.
From the start, you may choose to travel up or down. Either way that you choose, run to the very end of these hallways, and return to the first floor. When you arrive, the elevator that you just used will not return you to where you just came from. Instead, this elevator, as well as all three elevators that you find side by side (which are marked in the alternating gray diagonal lines), will take you to the third floor or second basement (where the black lines on the map flow to), and you will be stuck in the region of the fortress that is impossible to escape. Do not access any one of these elevators. Instead, run to the end of the first floor hallway, and again, travel up or down one floor.
Continue your journey to the end of the second floor or first basement and return to the first floor. At the end of the first floor hallway, you will reach an elevator that takes you down to the fourth basement. Traveling up on any of the fourth basement elevators is a risk that you shouldn't take. Instead, you want to reach the elevator with the red "f" on the bottom, and travel down. This will deposit you on the only good section of the second basement.
If it is your first time arriving there, you will want to press up again immediately. Instead of returning to the fourth floor, you will find yourself at the location of the password disk. After collecting it, using the elevator at the end will return you all the way to the start. You must repeat the same steps you used to reach "f" to continue. The next time you arrive at "f", run to the end of the hallway to access the elevator that will take you to the boss.
Note: If you did not rescue Iris, and collect every one of the password disks, including the eighth disk in this fortress, you will be unable to harm this boss with any weapon.
You might get excited when you find out that the final boss starts out looking identical to the seventh fortress boss. And in this form, he is just as easy to defeat as the previous boss. However, in this case, the first battle is just a warm up. After defeating the first form, this boss becomes metallic, and the head separates from the body. The head begins to float above the ground in a wave pattern, while the body continues to roam across the floor. Only the head shoots orbs of energy at Layla, the body simply tries to crash into you. You must defeat both the head and the body to win the game. Since the body stays on the ground, it is easier to defeat, and removing it makes fighting the head substantially easier. While avoiding both the head and the orbs, continue to fire at the body until it is defeated, or jump over it when it gets too close. Once the body is removed, begin jumping in the air when the head is a safe distance away, and attack it. The Flame Thrower is a good weapon to employ, even though it keeps you at a fairly close range. Do your best to attack the head in mid-air while avoiding the energy orbs and you should be able to destroy the head. At that point, the doctor will fly away, and you can escape the fortress in the usual manner to complete the game and watch the ending.