Floating Castle
With the CUBE in hand, it's time to return to the Mirage Tower and continue to the Floating Castle. You'll have to climb three floors and fight the Blue D each time, but by now you can do this by spending a minimum of resources.
Most of the Floating Castle's floors have a simple layout. This structure was built for function, not as a maze. There is a ton of treasure to be found on most floors, and you don't want to miss it. This is a very straightforward dungeon that should be much simpler than some earlier dungeons. Use EXIT if you get into trouble, but otherwise just keep climbing and keep your HP up.
Floating Castle 1F
You will appear on the first floor in a central room surrounded by a hallway. There are passages in all four cardinal directions. To the north lies the teleporter to 2F, and in every other direction lies treasure. Many enemies in the area return from previous dungeons, such as the BADMEN, MANTICORS, and NITEMARES, but it's the new enemies you need to concern yourself with.
You will run into EYES from the Ice Cave, though they pose little threat now. Your Warriors' HP should be high enough to negate XXXX even if they don't have Ribbons or ProRings, and they go down in one solid hit. GrNAGAS are hardy Spellcasters, but they don't use any powerful spells.
The most common dangerous foe are the EVILMEN that appear with NITEMARES. The look like BADMEN, but are powerful Spellcasters that can cast NUKE. Fortunately, it is their second spell, and they only decide to cast on 1/4 of their turns, but they should be taken out first in any case. GrMEDUSAS are a joke, especially if you have resistance to Fire and Poison. One shot with the Mage Staff will make short work of them. The Mage Staff is also useful against SLIMES, a throwback to the physical-immune SCUMS of the Marsh Cave. Only Fire will make a dent in them.
There are no treasure guardians in the Floating Castle, so collect loot at your leisure. You'll find the Bane Sword in the southern treasure room, a fun weapon that casts BANE for free. This isn't likely to win you many fights (with one major exception coming up), but it's useful if the wielder has nothing better to do in a round. The eastern treasure room features a second Heal Helmet and a free ProRing in addition to a good sum of Gold. The western treasure room offers more Gold and a simple HEAL Potion.
Floating Castle 2F
The second floor is more of the same: this time you teleport in from the north and will find the teleporter to the south, and there are rooms in eight directions instead of four. The monsters here are on par with the floor below. You will now see single CHIMERAS from the Mirage Tower, and the new RockGOLS are a rare encounter. They cast SLOW and resist magic, but wilt before a physical attack.
The biggest prize on this floor is the ADAMANT, which can be given to Smith the Blacksmith in the Dwarf Cave to be forged into Xcalber. You may even want to cast EXIT as soon as you find the ADAMANT in the southwest room to upgrade your KNIGHT's attack.
Going clockwise from there, you'll find two chests totaling 13,880 G in the western room. The northwest room contains a HOUSE and a simple Silver Helmet. Continuing on the east side, the northeast room holds one chest with an Opal Gauntlet. Directly to the east you will find the third and final Ribbon, along with a second Opal Shield. With three Ribbons and a KNIGHT using the Dragon Armor and Aegis Shield, your party would be resistant to virtually everything.
The final treasure room, to the southeast, contains the White and Black Robes. These are interesting pieces of armor that cast INV2 and ICE2, respectively. Both are quite useful, but by this point you are likely running very low on spare armor slots. These pieces are equivalent to Gold Bracelets in terms of Absorb, with a slightly higher penalty to Evade, and resist two elements apiece. The resistance isn't useful if your wizards are wearing Ribbons, but these are still pieces you want to keep. I wouldn't replace the Wh.Wiz's Opal Bracelet, but it's not a bad idea to drop a ProRing if you need somewhere to put the robes. A MASTER is especially useful in this regard since they tend to use less armor than anyone else.
Floating Castle 3F
The third floor has the most chaotic layout of any in the Floating Castle, but it still basically features a central area and four spokes off in the cardinal directions. The window in the center offers a hint as to your final destination. You will find the teleporter to 4F by heading west and then south.
There are several new encounters on this floor. SENTRIES sometimes appear with a GUARD, but don't think that they have similar capabilities. The SENTRY has double the HP and its attack is one of the most powerful in the game. MANCATS return from the Castle of Ordeal, and are still frustrating to fight. You should be well protected from FIR2 by now, which will blunt their attack considerably. R.HYDRAS appear in groups of four but are only threatening to Warriors lacking Fire resistance.
The rare encounters here are interesting, but nothing to worry about. WzVAMPS hit hard, but often waste turns casting ineffective spells. Their AFIR spell is bugged and doesn't actually work at all. FIGHTERS are cut from the same cloth, casting a variety of defensive and healing spells but offering little in the way of offense. They are support mages, and don't do much on their own, although they do tend to gain surprise over the party.
This floor features a lot of treasure spread out over its different areas. You'll find some interesting architecture, but no loot, to the south. In the room past the teleporter to the west, you'll find the Katana, a very nice upgrade if you have a NINJA in the party. The north treasure room has nothing of note unless you need another free ProRing.
The most interesting treasure room is the one to the east. You can reach it by bearing south as the path splits. The prize here is the ProCape in the southwest chest. This is technically a shield, but is usable by any class except the MASTER. 8 Absorb is a nice upgrade, especially for your Wh.Wiz, but it will cost you an armor slot you're probably using on some other nice piece of gear. You'll have to decide if you are willing to drop a spell-casting piece like a Heal Helmet, or to forego better armor (such as the Opal Helmet) to wear such a piece. Note that the chest directly above the ProCape contains mere Cloth, which is seemingly intended to get you to drop a useful piece of armor in favor of trash. Don't fall for it.
The most direct path to the end is to head east twice and south twice.
There are a variety of resource-draining and dangerous encounters on this floor, so don't spend more time than you have to wandering the endless halls.
Floating Castle 4F
This is a truly strange floor, featuring endlessly repeating terrain with no landmarks or treasure. You can walk forever in any direction, and it's easy to lose your bearings when combat begins. The quickest path to 5F is to go two intersections east and two intersections south, though alternating between north and west will eventually work as well.
While you wander around, you will be attacked by many of the same tough monsters you've already seen. New are groups of AIRS, tough opponents with a lot of HP and very high Evade. Their Magic Defense and lack of weaknesses make spells a less than optimal offense as well. Oddly, AIRS are the only elemental you can run from. How that works physically, I don't know.
The other new encounter is against one or two NAOCHOS. They are dangerous to any Warrior lacking armor. Far more dangerous are SORCERERS, which appear alongside MudGOLS. The MudGOLS' FAST spell makes them even more dangerous, and there is no way to resist their KO attack nor TRANCE. That said, your Magic Defense is high enough that both are unlikely to work, and SORCERERS may even run from the party at this point.
Floating Castle 5F
The top floor is a simple bridge to TIAMAT's lair. There are no SENTRIES here, so you only have one thing to worry about: WarMECH. This is a relatively rare encounter, but you should keep your HP up between battles on this floor just in case. You can run from WarMECH, but it often gains surprise and can dish out a whole lot of damage in a single round, let alone two. WarMECH is basically the series' first "super boss," and if you want to destroy one, you should consider waiting until a bit later in the game. This is a winnable battle, but will drain resources you'd rather keep for TIAMAT. For more information, see the WarMECH guide.
TIAMAT
TIAMAT is a tough boss with 1,000 HP and an unbelievable 80 Absorb, but she has several weaknesses you can exploit. She has two forms of offense: a four-hit attack that can dish out serious damage to anyone not wearing heavy armor, and a cycle of Dragon special attacks. FOG2 and the White Robe will reduce TIAMAT's physical damage substantially. If you're protected from the three primary attack elements and Poison already, you won't need to worry much about her special attacks, or put up any defenses against them. If necessary, use appropriate A-spells to start the battle. POISON can only be resisted with a Ribbon, the Aegis Shield, or the WALL spell, and is second in TIAMAT's cycle of four abilities.
As usual, FAST is the basis of a solid offense. Critical hits will deal most of your damage in this fight, which means your MASTER is especially useful. TIAMAT resists every attack element, so damage spells other than NUKE and FADE are a waste of time. With the proper equipment, you should be able to resist TIAMAT's attacks long enough to finish her off.
If you're impatient or just like to be mean, there is another option in this fight. TIAMAT is weak against Poison. Both BRAK and BANE have a chance of KOing her immediately. BRAK is more likely to work, but you can cast BANE for free with the Bane Sword as many times as you like. It's hardly fighting fair, but it is a lot of fun.
Once TIAMAT falls to your assault/cheap exploitation, the light will be restored to the final ORB. The Circle of Sages in Crescent Lake can tell you what you need to do next.