On the way to the castle…
You may be anxious to reach the castle and finish Zouna once and for all, but there are a few points of interest that might be worth checking out. There are two hidden shops nearby. If you follow the south coast of the 3rd continent back up and around to the east, you will find a small island north of the invisible maze. The hidden shop next to the dock sells the following: Gold Key (1600), Super Potion (320), Super Antidote (240), Power Ax (1200), Power Sword (1600), Super Tent (4000). There is another hidden shop on an island to the north west of Zouna's Castle, and it just happens to sell the same exact items. To the east of that island is a smaller island with a treasure chest on it. The chest contains a Tent.
Upon your arrival
You will disembark on the south west corner of the fourth continent. A large number of the poisonous gray flowers grow around here, so be careful where you step. Your goal will be to travel up and around the island in a counter clockwise direction. You will pass above the castle to the right and back down until you can see a gate. The gate requires a key. In fact, you will need the key no less than 6 times in Zouna's Castle. While it may seem preferable to have a Gold Key, inventory is very tight at this stage of the game, and you can't get rid of the Gold Key the way you can the regular key. If you insist on carrying the Gold Key, then you may have to leave the castle and return once you collect the Super Sword.
Breaking and entering
After you use your key on the gate to reach the castle doors, simply enter the castle to begin the final dungeon. There are a number of items that you can collect, and a number of ways to go, but only one direction takes you closer to Zouna. When you enter the castle, you will want to head right in order to proceed. If you intend to beat the game, and do not have a Sandra Soul, there happens to be one that you can collect in the castle, but you will need an Ax to get it, and it should be a regular Ax so that you can use it up, but it's not very practical. It's better to come prepared with a Sandra Soul already.
First order of business
You will need to get your hands on the Super Sword if you wish to beat Zouna. After traveling right from the entrance room, you will be forced to take a zig-zagging path down and to the right until you are finally permitted to travel up. Eventually, you will hit a locked door. Use your key to open the door to the room, as well as the door you will find on the other side. Simply continue to travel north until you find a room with an exit ladder. Turn left from here, and follow the path until it winds back to the north. After the next long north corridor, travel left and follow the rooms until you find a room guarded by a white Enmakonda. Defeat it and enter the room to collect the precious Super Sword.
Now, if you have no more free spaces in your inventory as a result of collecting the sword (presumably because you were carrying your current weapon and a Gold Key), and you are not carrying a consumable item (such as a tent or a Cure All,) you will have no choice but to leave the castle through the exit ladder that you passed along the way, and sell one of your items (again, presumably the weapon you were carrying) and return to the castle. You might as well return to a hotel and rest up before you finish the game.
The final challenge
From the Super Sword room, you will retrace your steps, and travel through two more long north-bound corridors. After the second corridor, enter the room to the left, and follow the path of rooms that follow. Eventually, you will be lead to a cavernous room and that opens up to the north. You are now in Zouna's chamber.
You will be attacked by several White Robotians, who can't hurt you physically, but fire off a lot of fireballs at you. Continue to travel up and you will see clocks and stone faces arranged in a circle, quite like a clock. In the center is a large throne and when it is fully in view, Zouna will warp in and proceed to attack you.
He appears on the throne, and teleports to your location. He will stand still and throw very large fireballs at you from time to time. Your goal will be to strike him near the head with your Super Sword. He will fly away from you as a result of the strike, and then teleport closer to you, but not as close as he previously was. Step forward a little bit and strike him in the head again. Reapeat this until he disappears.
Even though it looks like he dies, Zouna will instantly reappear on the throne and challenge you again. He will do this forever.
The trick is that sometimes when he disappears, he drops the one precious article he has in his possession: the Time Key. You need to pick it up. If you have no space left in your inventory, and one of the items is consumable, like the Cure All, then use it up now in order to free a space. Fortunately, like all items, the Time Key won't disappear (unless you walk so far away from it, that it leaves the screen). Once the Time Key is in your possession, walk up to and touch one of the three black key1holes that appear beneath Zouna's throne. As long as you have the Blue Mantle, Blue Helmet, Super Sword, and Sandra's Soul, pause the game, highlight the Time Key in your inventory, and use it on the keyhole to win the game. If you are missing any one of these items, you cannot use the Time Key. Congratulations on defeating Zouna and saving Marvel Land.