General hints
The castle looks very much like the castle of Maze of Galious. Use this to your advantage.
But there are differences as well. Most notably barriers so you can't enter all parts of 
the castle at the beginning. Also the worlds are in different places.

There are a few secret rooms. The first one is parallel to the room where key 1 is. Here 
you find the map room. Also there are two secret rooms in the middle tower which aren't 
in Maze of Galious. In one is the entrance to world four. The other has been found but 
it is still a mystery as to what you need to do there.

Saving costs 10 gold and will give you some magic and life back as well.


Big Boss was found as a text in the game's sectors. This might be a special password, but don't know how to use it.

Items

Key 1: opens world 1. Key 1 can be found where you find the cross in Maze of Galious
Key 2: opens world 2. You get it when you defeat the boss of world 1.
Key 3: opens world 3. You get it when you defeat the boss of world 2.
Key 4: opens world 4. You get it when you defeat the boss of world 3.
Magic Belt: use unknown. You find it 1 screen above the screen Aesculapius is in in Maze of Galious.
Red potion: Slowly restores vitality. If used too much you will lose it, but you can always get it again. You'll find it two screens left of the barrier between the right and middle tower.
Yellow potion: slowly restores magic. If used too much you will lose it, but you can always get it again. You'll find it in the right tower.
Priest's robe: lowers a ladder if you pray near a small cross in the left tower. You'll get it if you finish off all the bats in the room on the top floor of the right tower.
Bomb: select it to dispense with the barrier which separates the middle tower from the right tower. You'll find it one room below the red potion. Note that you need to hit the chest many times.
Magnifying glass: with it you can read the inscriptions on the tombstones. You'll find it in the southeast of the castle in the left most room with lava. 
Sand clock: slows down the rate at which the wall closes causing a barrier to the right tower. Can be found where you find the harp in Maze of Galious.
Magic spiral: allows you to make a double jump once. You'll lose it afterwards, but you can always get it again. Necessary to get past the barrier leading to the left tower and to get the vertical knife.
Cross: you get it by taking the magic spiral to the room below the center of the castle. It's the room where there is a blonde guy who says that there is a potion that makes you stronger. In the top floor of the room on your right jump up. Here is a secret room with the cross in it. Use the magical spiral to get to it. Use the cross in world 4 where you see another cross. You will have to beat the bats first, then avoid all kinds of bullets shot at you and finally vanquish some ghosts. If you do you get the crown.
Crown: with the crown you can open the way to the door with a crown above it.

World items (you lose them when you leave a world)

Bridge: place it over big lava pools, pits with spikes and so on to get across
Magical ladder: use it when there is a step missing in a ladder
Umbrella: prevents raindrops from falling
Head key: opens the door to the boss

Equipment

Sword power: allows you to break certain walls like the one leading to the map room and to enter the right part of the castle. Can be found where world 4 is in Maze of Galious. You need to talk to a man in the lower left corner of the castle after you find it to actually take it with you.
Dagger power: allows you to throw the dagger farther. Can be found one screen above the screen with the barrier to the left tower.
Fire control: allows you to control the direction of your fire blasts by heading in a certain direction yourself. Can be found in the room in which Aesculapius resides in Maze of Galious.
Fireball power: allows you to throw the fireball farther. You'll find it two rooms above the one in which you find the ring in Maze of Galious.
Feather: change the direction of your jump in mid air. Can be found in the same room where you find the feather in Maze of Galious. 
Wing: Allows you to walk a bit faster. In the same screen as the gold shield in Maze of Galious.
Boots: Allows you to jump slightly higher. Get it in the the screen above the one where the death shrine is located in Maze of Galious. Here you need to kill all creatures except for the not moving slime.
Shield: lowers the amount of vitality you lose when you get hit. Find it in the room where you get the bracelet in Maze of Galious.
Flamesword: without it you can't throw fire(balls). Find it in the bat pit one screen below the one in which you find the dagger in Maze of Galious.
Little bells: small animals are afraid of it, but exact use is unknown. Can be found where you find the green armour in Maze of Galious.
Trumpet Shell: Raises your chance of getting a drop when you kill an enemy. It's located at the same place as the earrings in Maze of Galious.

Weapons

Dagger: throws a dagger horizontally: lower right corner of the castle
Fire: throws three separate balls: in the right tower, two screens east of where you get the priest's robe in Maze of Galious.
Fireball: throws a fireball: room southwest of the feather
Vertical knife: throws knives vertically. One room west of the room in which you get the vase in Maze of Galious

Unknown items:

Key 5
Key 6
Key 7
Magic carpet
Red herbs
Yellow herbs
Red helmet
Blue helmet
Yellow helmet
White helmet
Fire bowl
Candle
Doll
Bell

Probably key 5, 6 and 7 aren't used in the game since world 4 is the last world.