The Northern Mines is located directly north of Trademond. In order to unlock it you must join the Adventurers' Guild and accept the first mission, Spider Infestation.
The Mines are made up of several maps, of which only the ‘top’ one connects to the World Map.
When you first explore this dungeon you will be accompanied by Charlotte, John and Paul.
If you go right there is a vine that you can climb up to a small cliff where there is an Ether in a chest. The rocks leading to the mines can be destroyed by a bomb (the easiest place to find one is in the Forest of Runes) in addition to by Star Knightess.
When you first enter the mines they are full of spiderling enemies. Defeating the young spider queen boss makes them disappear. The enemy groups consist of either three spiderlings or eight spiderlings. The groups of three can, if you have Charlotte use Fire, be killed before they attack, but the larger groups can do a lot of damage to your team.
The area is formed like a great circle and the young spider queen is slightly to the left of the entrance, but there is a gate that can only be opened from the far side blocking the way there, forcing you to go counterclockwise around the circle. To the left of the entrance is essentially a useless dead end until you open the gate, and there are several large groups of enemies. The enemies near the entrance is just a group of three though. Just to the right of the entrance is a small room with a body with some gold on it. When you finish the quest Spider Infestation (killing the queen and turning in the quest) the body gets removed and the gold is lost if you haven't grabbed it. Just north of this is a larger room with two groups of wandering enemies, one consisting of three and one of eight spiderlings. Down and right from this area is a destructable wall leading to a harvestable pyromantium and down and left (but above the way you came) is a harvestable ether. Both places have multiple groups of enemies and there are no reasons to go there before you've killed the queen to remove them. To the upper left is an enemy (eight spiderlings) patrolling rapidly back and forth in a straight line. It is absolutely possible to get by the enemy without fighting. From there is an upper and a lower path, both go to the boss chamber. The lower path has one required fight against three spiderlings, but then its just two groups of enemies, one three and one eight spiderlings. The upper path has no required fight but six groups of enemies to get past.
Either way you get to the spider queen's boss arena. There are four torches around the arena. If you light all of them before fighting her she gets a -75% defense debuff when you fight her. In order to make it easier to avoid fighting her before they are lit you can use the fact that enemies, including bosses, cannot see you while a dialogue is open. Interact with a torch and then wait to light it until the queen has moved to the far side of the arena.
In the center of the circle, accessible from the room between the entrance and the boss arena, is a room with an entrance leading to the cave-in area. The entrance is not active and cannot be used until you accept the Spider Cleanup quest, which requires you to have completed Spider Infestation. When you do four new groups of enemies spawn in this room, two groups consist of two spiderlings and two groups of one spiderling and one mature spider. The two can be told apart because the groups with a mature spider move around much, much faster. All these groups as well as the ones in the cave-in need to be killed to complete the quest Spider Cleanup.
A small area accessible from the central chamber of the Northern Mines once you've accepted the quest Spider Cleanup. The main room has four groups of enemies that need to be killed to finish the quest. Based on their starting positions, clockwise from the entrance they are: 1 spiderling 1 mature spider, 4 spiderlings 1 mature spider, 2 spiderlings 1 mature spider, and 2 spiderlings 2 mature spiders. In the upper right of the main room is a harvestable star metal. In the upper left is a destructable wall that can be broken through with a bomb or Star Knightess, leading to a small slime sideboss and a harvestable Pyromantium. Once the quest Demonic Vaults has been accepted you can use a rope here to go down to the Northern Mines: Underground.
This area is essentially a long corridor running left and right. You start to the south and the exit to the Vault is just north of you across the corridor, but it starts out locked. Between the way down from the cave in and the vault are two randomly wandering groups of enemies, one consisting of two spiderlings and two mature spiders and the other of four spiderlings and one mature spider. You don't have to worry about avoiding these too much as they are easily killed, Aura must have been able to kill similar groups on her own to even reach this area, and now she has a team. The remaining groups follow set paths and consist of two scorpios, a mature spider and a spiderling. These groups are much more dangerous and you should try your best to avoid them when you first enter this area. The scorpios can use pierce, which will deal significant damage due to its defense ignoring property. Paul's Pierce and Charlotte's Fire ara effective against the scorpios due to the scorpio's high defense, while John and Aura are more effective attacking the spiders.
To the right of the entrance and down is one of two levers that together unlock the door to the vault. At the top of the same room is a stair down to a small chamber where you find a formula piece, a Vitality Potion and a Mana Stone. To the left of the entrance is a longer corridor that eventually ends in small room with the other lever controlling the door to the Vault and a harvestable ether. Just before this there is an enemy patrolling up and down from a long one tile narrow corridor, following it will lead you to a small chamber with a chest containing three stasis bombs. It is perfectly possible to grab the bombs and leave without having to fight the enemy.
The vault is a fairly large area with a lot of content. The room you start in contains a teleportation stone that can teleport you to and from the Abandoned Shrine, or any other teleportation stone. Also in the same room are a formula piece and some gold. The path straight ahead leads to the boss, but there are curse traps in the way that will lower Aura's stats if you step on them, encouraging you to find another way.
All the (non-boss) enemies left of the starting room consist of one scorpio and three gnomes. All but one of these are stationary. When going left from the starting room you come to a room with two statues and a harvestable ether. The gate at the top of the room can only be opened from the other side. The skeleton in the top left has a mana stone. Taking the other path down from here brings you to a small room that then connects to a store room to the right. The enemy patrolling up and down in first room has to be fought, but the rest can be avoided. In the storeroom is a formula piece, 100 gold, an inzectoid armor and a rope.
All the (non-boss) enemies to the right of the starting room consist of two scorpios, a spiderling and a mature spider. Unlike the similar groups in the underground these groups wander randomly, making them much more difficult to avoid. Going right from the entrance chamber you find some gold on the ground and some gold on a skeleton. Going up you find some more gold on the ground and in a chest. Going even further up you come to a small underground lake. Here a young spider queen is guarding a red orb that can be used alongside a blue orb to open the door to the demon's anchor. In addition there is a harvestable emerald leaf, a formula piece and some gold on the ground. Going left there is a place where you can place a rope to climb down.
Climbing down this rope brings you to a chamber where a minotaur is guarding a gate leading north. The gate is opened by the two orbs and leads to the demon's anchor. Once you've learned Open Domain you can cast it to open an entrance to the demon's domain. Straight down from the minotaur is the curse-trap filled way back to the entrance chamber. Going left from the minotaur room brings you to the gate just left of the entrance room. There's also some gold on the ground and a destructable wall leading to a large harvestable ether deposit. Going even further left brings you to a small slime boss guarding the second orb needed to open the gate.
The Northern Mines were the private property of Arwin.
The mines used open torches instead of covered lanterns due to the cost.
The miners kept barrels of water along the mine in case of emergencies, such as gas explosions. 150 years before, many miners of the northern mines were killed when a fire broke out in the depth- destroying the supports and causing a cave-in.
The cave-in Aura discovers was caused by falling rocks, thus the reason why the mine was blocked off when she arrives.
While retreating from the attacking spiderlings, many of the miners used shovels as weapons, which kept many of the miners alive during their retreat out of the caves.