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Many things in this walkthrough are explained in video subtitles and also in the description below. Please read these before asking questions.
My complete Palace of the Four Sword playthrough for The Legend of Zelda: A Link to the Past & Four Swords. The alternate Dark World ending sequence is also included. Played with cheat codes on VisualBoyAdvance 1.7.2 re-recording v23.5 svn421.
NOTE: At first, I was planning on splitting this video into two parts, but I decided to combine them together instead. I also reencoded the video in High Definition (720p HD) format.
The Palace of the Four Sword is a bonus dungeon ONLY available on the Game Boy Advance (GBA) version of The Legend of Zelda: A Link to the Past. The official way to get in there is to beat both LttP and the multi-player Four Swords extra that comes with the GBA version (you'll have to play Four Swords with a friend who also has a GBA copy of LttP), but for this video I used a cheat to get past the fox guarding the entrance. If you want to use a glitch at Level 4: Thieves' Town instead, that's perfectly fine, too.
Palace of the Four Sword cheat code: 030038F0:20
Inside the bonus Palace of the Four Sword, there are rematches with the first four Dark World bosses (Arrghus, Helmasaur King, Mothula, Blind), except they now have new colors and are more powerful. The Four Dark Links (aka Doppelgangers) are the final bosses, and defeating them will award you with an alternate ending sequence with scenes from inside and outside the Dark World dungeons. Also included at the end is a Quest Log that takes into account your item usage, number of times you save your game, fairies used, rupees found, enemies slain, and your total play time.
Here are the time-stamped notes for the altered Dark World bosses and the final boss, as usual:
[01:34] Boss #1: Arrghus II
Arrghus is now surrounded by Tentacles that can electrocute you if you're not careful. His main jellyfish is also orange instead of red.
[04:51] Boss #2: Helmasaur King II
The Helmasaur King is now green with an orange mask. He also spits out mini Helmasaurs while his mask is gone. After you break his mask for the second time, you have to shoot a Silver Arrow at the eyeball that appears on his forehead.
[07:26] Boss #3: Mothula II
Unfortunately, Mothula now has two clones that are impossible to distinguish from the real one. He also generates new ones if you destroy both clones like I did. The best strategy is to simply kill one clone and let the bees that arise fly out to the real Mothula.
[10:00] Boss #4: Blind II
Don't be fooled if you see Blind disguised as Link's uncle; after he reveals his true form, you have to return his detached heads to him to inflict damage on him.
[12:13] Final Boss: Four Dark Links (aka Doppelgangers)
The Dark Links in this game are colored in green, red, blue, and purple, and all of them have their own abilities. The use of spin attacks is strongly encouraged.
Green Dark Link: This guy is pretty easy since he only uses regular sword swipes and the Pegasus Shoes.
Red Dark Link: Even though he starts using the hurricane spin attack, he is pretty easy to defeat.
Blue Dark Link. This one is a bit tricky. Not only does he use the downward thrust, but he also goes invisible with the Magic Cape to prevent you from attacking him. Stand to the side of him when he uses the downward thrust to avoid getting hit. He does not use the hurricane spin attack.
Purple Dark Link: He uses every attack method including regular sword swipes, the Pegasus Boots, the hurricane spin attack, the Magic Cape, the downward thrust, and also shoots fireballs.