To Castle Deist
To reach Castle Deist, you'll want to travel southeast from the strait leading to the outer sea. The easiest way to find it from either there or the Tropical Island is to head due south until you reach the north edge of the Snow Field, then follow that coastline east. When it ends, Deist Island will be just to the southeast. Don't forget to use -Select to find your way if you lose your bearings.
There are no amenities on Deist Island, so if you need to visit an inn or sell off extra loot, you'll have to sail elsewhere. The most convenient location is Bafsk, which is a short trip due west from the island.
There is one new Monsters on Deist Island, the Stalactite. With only two attacks and 240 HP, they are less of a threat than many of the Monsters found deep within the Tropical Island, and they have a weakness to Poison. You can make good use of the Scourge Spell you found in that dungeon against them. They typically appear in groups, but can also accompany Ogre Mages, alongside Ogres and Land Turtles. These Monsters are nothing new and you should be well-equipped to handle them by now.
There is an even less intimidating fight against various types of Undead here, but beware of the rare encounter with a Hill Gigas or two. They are as powerful as a boss, especially in pairs. Liberal uses of Spells like Berserk and Blink, as well as application of Curse with the Ancient Sword or the Curse Spell you'll find shortly, are recommended.
Castle Deist
You'll find Castle Deist in the center of the island, north of a small lake. While it is laid out like a typical dungeon, complete with trap rooms, there are actually no Monsters to be found in Castle Deist at all. You will first encounter a small boy who keeps running away, leading you to his mother and the last Wyvern on the 2F. Follow the nearby hallway south to them. The room they stand in front of contains a dying Wyvern, who you are unable to communicate with. The woman here asks you to find and return with the Pendant that was lost in the Deist Cavern, which will allow you to understand the Wyvern's language. This is your new priority.
There are 14 Treasures to find in Castle Deist with a bit of legwork. Keep in mind that your inventory is limited, and there is plenty of Treasure to pick up in the Deist Cavern as well. Since these are unguarded, you may want to come back later, or sell off any excess before heading there. You will most likely want to at least grab the Wing Sword and the new Spells, though. This is the first accurate sword in some time, with nearly identical stats to the Demon Spear (minus a few % of accuracy). If you need to resume training anyone with swords, this is the Weapon/Shield to use.
There is a Cottage in the northwest corner of the 2F. The stairs back to 1F in the northeast corner lead to the majority of the loot found here. Take the hallway west to find a room with 8 chests. From left to right, you'll find Eye Drops, an Antidote, a Cross, and a Mallet in the top row, and a Gold Needle, Sage's Wisdom, Saint's Spirit, and the Wing Sword on the bottom row.
The southern path from the stairs to 2F leads to a pair of rooms, the first of which is empty. The other holds three chests with Spell tomes: from left to right, Stop, Stun, and Curse. Stop is new, and while you could have found Stun or Curse as Monster drops, this is the first time you're guaranteed to see them. Curse in particular is useful for all the reasons the Ancient Sword has been during the game, bringing tough bosses' defense and attack down to reasonable levels.
At the end of the hallway with the Spell room, you'll find another chest, this one containing a Unicorn Horn. This useful item will cure all temporary Status Ailments for the whole party with Basuna 16, and has only a 10% chance to break. A mage in the back row can make use of this to quickly recover from dangerous Status Ailments inflicted by some monster attacks, like Confuse.
The final Treasure in Castle Deist is a Phoenix Down. To find it, head back to the 2F and take the southern path. It leads to stairs in the southwest corner, which in turn lead to a small room 1F with a chest containing the Phoenix Down.
Once you're satisfied with your haul, it's time to head north to the Deist Cavern. Your first task is t retrieve the Pendant, which will be a short trip, but you'll be exploring the Cavern fully soon enough.