Now that you have access to Magic Keys through Rimuldar, you can access locked areas in regions you've already visited. This page gives a full overview of these, though several of them are unnecessary.
There are two locked doors in Kol. To unlock one of them would be a terrible waste since you can freely access the other side. However, if you use a key to talk with the man behind the second door, he will ask you if you found the flute. If you say no, he'll tell you that the man who owns it left for Rimuldar. If you respond positively, he will tell you to head to Cantlin, a town you have yet to visit. But what is the purpose of this mysterious flute? One warrior in town lets you know that Golem is afraid of the music of the flute, although you have never encountered a Golem before.
When you use a Magic Key in Rimuldar to speak with the man in the Inn, he will have told you to search a location exactly four squares south of the bath. Should you happen to search in the location depicted in the image to the right, you will indeed find an item: the Fairy Flute.
There is one locked door in the town of Brecconary. If you decide to open the door to the building in the north east corner of town, you will find a shop tended by a woman. If you talk to her across the counter, she will offer Fairy Water to you for the price of 38 gold. Fairy Water prevents encounters with the weaker monsters like Slimes and Drakees away from you as you wander across Alefgard. This can spare you the constant interruption of battles that yield little experience and gold, but doesn't prevent encounters with tougher enemies. If you leave the town and come back, the door will be closed again, so buy a lot of Fairy Waters in advance if you intend to use them.
You learn the spell of Repel at level 15, which has the same effect as Fairy Water.
There are two doors to unlock in Tantegel Castle; the essential Stones of Sunlight are hidden behind one.
One door opens into a treasure room, where the guard inside will admonish you for taking what isn't yours. All that you will find is barely enough gold to make up for the cost of the key that you just used.
The door near the back of the castle is more useful. Enter inside and you will find a number of people. One man mentions that he can't find the entrance to the castle cellar. Another guard stands outside of a room that has an especially dangerous type of barrier tile, which will damage you 15 HP per step. The man on the opposite side says something of interest: You must push against a wall of darkness to find the grave of Garin in Garinham. Remember that and proceed back across. Heading north a bit, you will find an outdoor shop selling, of all things, Keys! However, at 85 gold per key, they will set you back a bit more than in Rimuldar. Still, it's easier to stock up here than travel all the way east again.
You've looked everywhere, and yet, there's still no sign of a cellar entrance. The cellar is present, but it is in a very unlikely location. Remember how you had to travel just outside of the moat in Rimuldar to locate the entrance to the key shop? You'll need to do something very similar. The girl standing to the right of the Tantegel key shop tells you about the legend of the Rainbow Bridge. You can walk around her and down the side of the castle. Eventually, you will hit water. Be brave and travel just one more square to the right, and you can continue around the body of water where you will find the stairwell entrance to the cellar. A man inside will greet you happily and invite you to take the contents of the chest behind him: The Stones of Sunlight.
Now that you have keys, you can finally explore that large building that occupies the north portion of Garinham. Stepping inside, you will find a number of people, as well as another odd site: a wall of darkness. Talking to the people inside, you will learn a few things: Edrick's Armor has been hidden, and there used to be a town to the south known as Hauksness. Do not waste a key on the door within the building as you will only encounter two bickering guards. The few treasure chests on the floor offer a herb, a torch, and a bit of gold. But is this all there is to this mysterious building?
If you braved the dangerous tiles that were on the other side of the door in Tantegel Castle, you would have spoken to a warrior on the other side who suggests that you push against the wall of darkness. There are no real visual indications of precisely where to look, so you must push against every square on the wall. Eventually, you will find an opening that was hidden from view. The opening leads to a path behind the building. As you follow it, you will come to a man who warns you of the evil of the Harp, but you can't afford to ignore it. Use the staircase to enter the Grave of Garin.
The Grave of Garin
In addition to any keys that you used to reach the grave, you will also need one more to proceed all the way to its depths. Keep a high MP count as well, as you will need the spell of Radiant quite a lot; it's far more useful for navigating dark corridors than a torch. It is highly recommended that you wait until at least level 12 before entering the grave so that you can make use of the Outside spell if you are in danger of dying. However, Outside requires 6MP to use, so if that's what you're down to, use it and rest up at an Inn. If you are at level 12, with Full Plate (or better yet, Magic Armor) and a Broad Sword, you'll be in good shape here.
Enemies that you encounter here:
Floor one is quite expansive, with a number of twisting and criss-crossing corridors. There are three treasure chests stored in an alcove to the north, containing a herb and scant amounts of gold. The real goal is to reach the locked door in the bottom right corner. Unlocking that door with a key will allow you to travel along the southern branch of the floor to the staircase that leads to floor 2. Don't worry; the door will stay unlocked as long as you remain in the dungeon. However, if you leave, you'll need to bring another key with you.
Enemies that you encounter here:
Floor 2 consists of an arrangement of five downward staircases, arranged in each corner as well as the center. The northeast and center stairwells lead to dead-ends. The northwest and southeast stairwells lead to corridors that connect to one another. The only stairwell that allows you to progress deeper into the grave is the one in the southwest corner.
Floors 3 and 4
Enemies that you encounter here:
Of the enemies present down here, the Wraith Knight is the most fearsome. With a strong sword arm and the ability to heal itself, fights may last a little longer than you wish. Wolflords can be put to sleep, but Specters like to Sleep you, so be ready with a Stopspell unless you're confident that you can kill them quickly.
As mentioned above, if you took the northeast or center stairwell, you will find yourself in a dead-end. If you took the southeast or northwest stairwell, you will find yourself traveling most of the outer edge of this floor. From here, you can access a single treasure chest: the Curse Belt. Whatever you do, do not inspect the belt in your inventory. It will wrap itself around you and weaken you if you do. Then the only way to remove it is to visit the curse-breaker in Brecconary Town. The belt will sell for a worthwhile 180 gold. The only way to reach Floor 4 is to take the southwest stairwell from Floor 2 and follow the north path to stairs that lead down. Floor 4 is very small, and leads you to a rising set of stairs. Climb them, and you will reach the chamber containing the Silver Harp. Now you may cast Outside to leave the grave, or simply work your way back to the top for the extra battle experience.
Once you have the Silver Harp, there are two things you can do with it. One is to use it in any outdoor or dungeon location. It will instantly summon a monster to the spot and you will be forced to fight it. This is fine if you're healthy and want to earn experience points. However, to use it for its true purpose, you must fulfill your end of the bargain with the man in the Kol Shrine. He will trade the Harp for the contents of chest behind him, which happens to be the Staff of Rain.