You start out in the Castle, at E-5 indicated by the yellow circle with the S inside. To complete the game, you must navigate through the Castle. Initially, you will only be able to explore a small portion of the dungeon. As you progress and collect useful items, more and more of the Castle will become accessible to you. The first item you will need to find is the Mongolian Sword, which allows you to break many of the blocks that lie in passages and prevent you from exploring deeper into the Castle.
The Castle contains the entrances for all five dungeons. Each entrance is marked on the map above by a blue circle with a number in it. As more of the Castle becomes accessible, more of the dungeon entrances can be reached. However, you need to collect a Great Key before you can enter them.
There are four other entrances in the Castle. Two of them are shops, but the other two serve important functions. The door at D-6 is the password room, where you can go to receive a password that will allow you to continue your progress if you turn the game off and come back later. The door at C-7 is a room of resurrection, where you can revive a fallen comrade. However, in order to enter it, you must first have collected the Salt item, and you must pay a price of 100 ammunition and 100 coins for the act of resurrection.
There are five dungeons in the game. They each contain a different arrangement of floors. To complete each dungeon, you must locate the room where the boss may be summoned, and then cast the spell which summons the boss. Summon spells are composed of various control pad inputs. There is always one room in each dungeon where a woman will teach you the required spell.
In each dungeon, you can also find three items which will substantially increase your chances of defeating the boss of the dungeon. They are the Mantle, the Holy Water, the Magical Rod. Each item gives you an advantage over the boss; reducing the damage they cause you, increasing the damage you inflict on them, and the ability to use weapons infinitely without drawing from your ammunition.
Once the boss is defeated, the next Great Key will fall from the ceiling, permitting you access to the next dungeon. But there are often many other valuable items in each dungeon, some of which must be found, some of which must be purchased, and many of which are required in order for you to progress through the Castle to the next dungeon entrance. Always take the time to explore.