You can only enter through the Time Door at night. Once you find the right location, as described by the man in the Prison, use the Statue of Varalys to enter. You need the Exit Talisman in order to escape from the Time Door, or you will be trapped inside until you either defeat Kaizack or die.
It will take a long time to get all the way through the Time Door to challenge Kaizack and save the world. It may take so long, in fact, that you may grow too tired to fight. If this happens, you should leave to stay at an inn and try again. Every enemy in the Time Door is evil and strong. Fight them carefully to reduce the amount of damage you take.
You start in the center of the first floor. From here, you can go in any direction. There is one treasure chest on the floor, located in the upper right corner. Inside, you'll find the last of the fairy equipment, the Fairy Shield. While it's quite powerful, it's also quite heavy. If you are struggling to hold everything, you may need to cast the Weight spell to enable normal movement speeds. The transport that leads to the next floor is in the upper left corner of the floor, and can only be accessed from below. The fastest way to get there from the center is to head straight down and follow the outer wall left and then up until you arrive at the transport room.
You begin in the upper left corner of the floor. To reach the next objective, you must head to the right, and make your way to the upper right corner before following a path down and then to the right. When you reach an area with a thin walkway, you must follow it to the right very carefully. If you fall off the walkway, you will die. Continue all the way to the right and examine the bottom of the large object in the middle. A computer will indicate that a fairy is being held captive on the third floor. You must do this in order to be able to find the fairy. The drop down to the lower path and head to the left until you return to solid flooring. The transport to the next floor is just above you.
You arrive along the right side of the floor. You can explore below you, but it dead-ends, so it's better to simply head up. Once again, be very careful not to fall to your death off the narrow walkways. The transport to the next floor is easily accessed from the very center of the floor. However, you'll want to explore the lower left corner of the floor, particularly along the left wall. Between the two walls that stick out, you should hear a chime indicating that you should search at your feet. If you checked the computer on the floor below, you will discover the fairy. The fairy will plead for you to rescue her. To do that, you'll need to head to the fourth floor.
The fourth floor has a few things going on. The transport to the fifth floor is conveniently located one room over to the left. However, there are reasons to explore the far left and right sides of the floor. The mechanism that controls the lock for the fairy's cell is found on the far left side of the floor. Examine the left wall until you hear the chime, and search the floor. Once you do this, the fairy will be freed, but you still have to return to the third floor and collect her. Without the fairy, you will be unable to see the final boss of the game.
On the far right side, you'll find a treasure chest. In this chest is camping gear. However, the tiles immediately to the left and right of the chest of cursed with a teleportation trap which sends you down to the second floor. The only way to safely claim the chest is by walking below it and coming up. Camping gear is very useful because it's like staying at an inn wherever you choose to use it. However, it's very heavy. If you have not had to rely on casting Weight so far, it's very likely that you would have to at this point. Your only other alternative is to use it immediately. Definitely don't throw it away.
Once you have retrieved the fairy from the third floor, head to the fifth floor.
You begin in the lower left corner of this floor. The structure of this floor is similar to the second floor. You must head up and to the right until you have to come down and head to the right. There are many trapped tiles on this floor, including poison arrows, and some that drain your health as you walk on them, so be prepared to heal. You have to walk all the way to the right in order to come down and bit and head back to the left. When you reach a room that appears to dead-end, you can in fact head down through the bottom to one final room.
It is in this room that you must make final preparations for the battle with the final boss of the game, Kaizack. If you have been relying on your Light Saber through the Time Door, it is time now to switch to the Flame Sword. Additionally, you should heal up and cast any other magic necessary because you won't be able to cast any more magic when the battle starts. Don't cast Invincibility, as the game detects that and does not allow you to harm the boss until the effects of the spell wears off.
There is one thing you can do to give yourself an advantage in the fight: speed up the game. For some reason, changing the speed of the game has no effect on Kaizack. Whether you move at the normal speed, or the fastest speed, Kaizack's speed will remain the same. Speeding yourself up will give you a fighting chance against him. When you are ready step onto the lower portion of the floor to be transported to Kaizack's lair.
Kaizack will appear, but you will only be able to see him if you rescued the fairy. Kaizack's behavior is simple. He will walk around in a large rectangle along the top of the room. As he does, he spits large balls of energy at you that are basically impossible to avoid. The only way to get him to stop doing that, temporarily at least, is to hit him in the mouth. Doing so will force him to pause shooting at you, but he will eventually resume his attack.
Kaizack has these four bumps at the top of each limb. In order to attack Kaizack, you actually need to aim your Flame Sword at these bumps. You can't get too close to Kaizack. Touching him will do a lot of damage and force you down to a lower level. Keep a safe distance from him and fire upwards at those bumps. As you do enough damage, each of them will get destroyed. Remember to shoot his mouth from time to time to decrease his rate of attack.
Once all four bumps are destroyed, Kaizack will freeze in place, but he won't stop shooting the energy at you. It is at this point that you should approach his mouth... and hop in. Kaizack will swallow you, and your health will begin to drain, but you will still be able to attack. From the inside of his mouth, continue shooting your Flame Sword. If you had enough health left over, you should be able to defeat Kaizack from inside before he completely drains your health. If you are successful, Kaizack will admit defeat, and you will see the ending of the game.