Now that you've made a pile of money on the sub mini game, you should return to the shop in the castle and buy the best weapons and armor for everyone. You should also stock up on Elixirs and Holy Waters (It's not a bad idea to have up to 30 of each, now that you're rich). If you want to save some money, but the potions from the shop in Flowey as they are cheaper there.
Ok, let's do this in a reasonable order instead of just marching all over the place. The level makes it easy. I recommend starting by heading out the door and turning left. You'll come to a little alcove. There are 4 of these, all alike, with a staircase going up and one going down. If you go down, you'll find yourself in a small underwater room with a chest in one corner. That's the case for all of the staircases going down. The staircase going up will take you up two floors to a chest in the top center of the room. Opening the chest reveals something of terrible evil: a giant carrot! Defeat all 4 carrots to collect 4 flints, which you'll use to light some alters.
After taking the first room to the left, go up and get the second flint from the second alcove. Then go right to the next one, and down to the next one after that. But while you're at the north end of the fortress, you might as well enter the two rooms on the end. The first one (if you're going in the suggested order) has a alter with a spot for a medallion. You did get the medallion right? If not you'll have to come back with it later. Place the medallion in the alter and a chest will appear. Collect the bounty (the Looking Scope) and head to the room on the other side of the fortress (the middle room is locked still).
The room at the north west end of the fortress is one you don't want to go bailing into. You must walk a certain course or you'll fall into the frigid waters through the floor. If you do fall, it's recommended that you use Cure3 immediately on your whole party, as another fall will mean death if you don't cure up. The safe paths are laid out in the shot below.
The Mystic blade is very special and can only be equipped by Eny. Do it. Or if you want to see what happens if it's not equipped, save your game before unlocking the middle door at the north end of the fortress. But to advance beyond the fortress, you will eventually need to equip Eny with the Mystic blade.