Don't be fooled by the idyllic scene presented at the start of this stage—this is a tremendously difficult stage, and it starts out with two octopuses. Your instinct may tell you to fight them and remove them, but don't be so quick. If you happen to have really good boots, or even Winged Boots, do not kill them. You will benefit from the game's limit on the number of enemies it will place on the stage, and be able to reach part B with minimal conflict.
If you don't have the Winged Boots, you'll have to remove the octopuses, which will cause two spitting plants to appear on top of the roof of door A. They are incredibly difficult to approach, much less defeat, and they will spit thorns at you constantly.
You'll also have to defeat two skeletons before a moving platform will appear to transport you to the island to the right. If you manage to get Winged Boots from one of the enemies, you won't have to wait for this platform, you can immediately jump ahead.
Beyond the next island is a small island, but your destination is actually in the water. Dive down between the last two islands to reach part B.
If you intend to visit the shop behind door B, be sure to hold left as you fall through the water. If you hit the bottom first and your shoes aren't good enough, you'll never get back up there. You'll especially want to do this if you happen to have the charm from Pororo Island
The shop behind door B is nothing special, but if you possess the charm, it hides an important secret. Enter and exit the shop two times, and the door will be nailed shut. Push up to enter the door despite the wooden planks, and you will be allowed to enter. You will meet the Great Catfish, who will reward you with the Hero Emblem. It won't take you long to find out what to do with it either.
Once you're done with the shop, you'll need to press on to the right. After a crab, a series of octopuses will keep you at bay by spitting blobs at you. You're best bet is to do nothing and leave them alone, or the game will replace them with more octopuses for you to deal with further on. Try your best to leap over them without fighting them and make your way to the far right end to reach part C. If you do defeat all of the enemies along the way, you may wind up facing a Mushroom King near the end.
It certainly doesn't get any easier in this cave. You'll start off facing a Mudman who happens to have a Kong standing behind him, chucking boulders at you. Do your best to remove both before continuing on.
Along with another Mudman, you'll find a Poverty waiting for you to approach. Deal with both as carefully as you can, then approach the platforms to the right.
In order to survive this very difficult section of the stage, it is important to only kill one of the two spitting plants which appear when the level begins. If you do this, it will keep the pace at which other enemies appear at a manageable level. Alternatively, you can ignore both and try to dash through the entire area, although you may miss useful power-ups.
As long as you only killed on plant, the remaining plants will appear one at a time as you proceed to defeat each of them. They primarily drop point items which may not be too useful at this point, but they sometimes drop hearts as well.
Further down you will encounter a set of four Strong Bats. Do your best to deal with these carefully before dropping down below. Two more plants await you below.
After the last two plants, you won't encounter any more resistance to the bottom. Just as in Round 9, this vertical sections ends in lava on the left and the escape to the next section on the right, so be careful when dropping from the ladder.
This is the final approach before you reach the gate that leads to the interior of the Castle of No End. It is guarded by a cadre of red knights and one blue knight (you will encounter several short blue knights in the SMS version.) Attacking the knights can sometimes yield useful items, but it may also lead to the loss of precious health so judge the situation accordingly.
There are more doors in this section than any other section in the entire game. However, a majority of the doors are actually hidden. You need to press up as you walk along the castle exterior hoping to hit a section containing a door.
The first visible door, C, leads to a bar. However, there are two useful hidden doors just beyond it. Door D is an armor shop, and it is your very last chance to obtain the Legendary Armor. Although expensive at 540 gold, you will absolutely not want to miss this item before you enter the castle. Come back if you have to. Likewise, Door E is your last chance to purchase a potion if you don't have one, although you could get lucky and win won off of a defeated enemy as well.
Door F is a hospital and it is highly recommended that you stay here if you already have a potion and have less than five hearts. Door G is a shoe shop and your last chance to obtain Legendary Shoes. However, as long as you have Leather Shoes, you should be alright. Only splurge on the Legendary Shoes if you already possess Legendary Armor.
Door H leads to the boss, which you can read about below. Door I is a shop that usually has useful inventory items. Door J is possibly the most important door of all. If you managed to succeed in every part of the item trading subquest, all the way up to the part where you traded the charm for the hero emblem in part B of this round, you are now presented with a useful but important choice.
Inside door J, you can exchange the emblem for one of two powerful items. One is the bell (left choice), which will help you by indicating the correct way to get through the Castle of No End. The other is the ruby (right choice) which will do significant harm to the final boss of the game. You may only choose one, so make your decision wisely. In general, if you know your way around the castle, you should take the ruby. Otherwise, take the bell.
The knight that you face off with at the end of this stage is no ordinary knight. It is the one and only silver knight. It is more powerful than a blue knight, primarily due to it's speed and leaping ability. Power aside, your strategy against this knight should be no different than any other. Press your attack and force it to leap over your head when you push its back against the wall. Then turn around and continue to press your attack until it is defeated. There is no need to use special weapons—save them for inside the castle.
Even though it drops a key and eight money bags, you are not finished yet. There is still at least one more useful door to explore. When you are truly ready to proceed, head to the gate with the key and enter the castle.
