The higher in level you get, the more instanced dungeons you can enter. Instances are meant for groups, and you’ll get killed when trying to do quests in there by yourself unless you are way above the intended level of the instance. All instances have multiple quests associated with them, some more than

others. If you are doing an instance with a good group, you will find experience to be flowing much faster then normal, and cash and items come much more quickly. If you run short on good quests, you always have the option of doing a few instance runs for exp, gold and items. I found that if you want a good group, form it yourself and preferably with people you know. That way you can decide the level and classes you want to enter the instance with.
Alliance
The Deadmines (15 – 25)
In the town of Moonbrook in Westfall. The Deadmines has an area outside of the actual instance, you will need to fight a bunch of miners around level 15 to get to the portal.
The Stockade (23 - 33)
Location: The center of Stormwind. A small Instance, not particularly difficult.
Horde
Ragefire Chasm (14-20)
In Orgrimmar’s Cleft of Shadow. This is a good beginner’s instance, it is nice and small and doesn’t contain any overly complex fights.
Wailing Caverns (15 - 25)
The entrance lies in a mountain just southwest of the Crossroads outpost in The Barrens. The Wailing Caverns are divided into an outside area where anyone can hunt, containing the lower level end of the elite monsters. A portal inside will take you to the instance portion. There is a secret area above the entry to the zone which you can reach by climbing the mountain and dropping down the middle of the large skull that makes up the entrance. Several quests are offered in the right eyesocket.
Shadowfang Keep (20 – 30)
Location: Silverpine Forest. South of The Sepulcher, just north of Pyrewood Village.
This zone is a step up from Wailing Caverns. It is relatively small, and has some very nice loot drops for warlocks.
Both Factions
Blackfathom Deeps (22 - 32)
Northern tip of the Zoram Strand, the western coast of Ashenvale. A large instance, not as frequented as the others around this level. Several good quests and fun to do.
Razorfen Kraul (30 - 40)

Located in the Southermost tip of the Barrens, west of the Great Lift. This instance is split in two parts which do not connect. A short ways from the entrance you can choose

to between going down a ramp to your left or heading straight on. To the left is where the Redleaf Tuber quest can be done, and deeper in an escort quest can be done.
Going straight on leads to the majority of the zone, and contains the most named. Doing both parts in one sitting will generally take a long time, as this is a pretty large zone.
Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40) Gnomeregan (30 - 40)
Gnomeregan lies in Ironforge. Horde players can teleport to the instance from Booty Bay after completing the ‘Chief Engineer Scooty’ quest, gotten from Sovik in Orgrimmar at level 30 or higher. Watch out for the small alarm bots here, they must be killed quickly, or they will summon reinforcements.
Razorfen Downs (35 - 45)
Location: Southern tip of The Barrens, east of The Great Lift. Just a few levels higher then Razorfen Kraul, this mazelike zone contains alot of undead.

Scarlet Monastery (35 - 45)
This zone lies north of the Undercity in Tirisfall Glades. It is split up into three separate instances to allow you to do it in more then one visit without having to redo the entire zone. Ranged from easiest to highest, there is the Library, Armory and Cathedral. You will need a key found in the Library to open the doors leading to the Armory and Cathedral.
Uldaman (40 - 50)
Situated in the south of the Badlands, right by the passage to Loch Modan.
Many of the Uldaman quests take place before you enter the actual instance, and are easier to do then the ones inside. The instance itself is quite large, with a cool boss event at the end.
Zul’Farrak (45 - 55)
Zul’Farrak can be found in the northwestern area of Tanaris. It isn’t a very large instance, but has some difficult events which can slow a group down considerably.
Maraudon (45 - 55)
Maraudon lies in a mountain pass just northeast of the Troll outpost Shadowprey Village in Desolace. The pass can be accessed from the south and east. The instance consists of two main paths, each with their own entry. Before you enter the zone, you will see an orange and a purple tunnel, each leading to a zonein portal. The orange one is the more difficult of the two, but will let you fight different bosses and complete quests you wouldn’t otherwise be able to do.
The Temple of Atal’Hakkar aka SunkenTemple (50 - 60)
The temple lies in a lake in the Swamp of Sorrows. You enter it at the top, then go down through an underwater passage and have to fight a way in until you reach the zonein.
When you zone in, there are three ways to go. None of the ways are interconnected, so you may choose to only do part of the instance and come back for the rest later without having to reclear the whole zone.
Blackrock Depths (53 - 60)
Enter it inside the Blackrock Mountain, which lies south of Searing Gorge, north in the Burning Steppes. It contain three different instances. BRD is found all the way at the bottom (use the giant chains to go down) through the quarry.
This is a very large instance involving many quests, some of which are chainquests.
Lots of good loot and cool events here.

Dire Maul (57 - 60)
Found in the ruins in the center of Feralas, north of Feral Scar Vale and west of Camp Mojache, Dire Maul is split into three parts. The eastern instance, which has a main and a locked back entry in the form of a stone structure north of Camp Mojache, contains mainly satyr demons and plants. The satyr make good pets. You will need to complete an event with the imp at the main entry, pim’gib, to obtain the key for the other two instances and the back entry to the east one. The western instance contains mainly elementals and undead. It is also involved in the last step of our dreadsteed quest.
The northern instance is the toughest of the three, filled with mainly ogres. Lots of named and good loot to be had here.
Stratholme (57 - 60)
Lies in the northwest of the Eastern Plaguelands. A back entry requiring a key from inside the zone is found to the east of the main entry.
Stratholme is a high end instance, most people wait until 60 to go here. It is split into two main area, live and dead side. The live side mainly deals with Scarlet Monastery humans, where the dead side has, surprisingly, mostly undead inside. Many quests involve stratholme, and it is a great zone for loot.

Scholomance (57 - 60)
Scholomance lies on the island of Caer Darrow, southeast of the Western Plaguelands.
It has one main path, you are required to clear most of the zone to get to the end.
It is about as difficult as Stratholme, and the bosses drop loot to match the difficulty.
Blackrock Spire (57-60)
Found in Blackrock Mountain, through a sidepath in the main spiraling pathway inside the mountain. A bunch of orcs will need to be dealt with before entering.
This zone is split into two areas. As you zone in, you can choose to go up and left, or down and right. Up (UBRS) leads to the raided area, meant for 15 people, and requires a quested key to enter. Down (LBRS) is doable with 5.
LBRS is a large zone to clear with a single group, and usually takes a few hours at least. It is stacked with named and good loot.
UBRS is done with 15 people, and requires cordination and some organization to do effectively. It has the best loot, but is more difficult then LBRS to do. It is also involved in the last step of the Onyxia Key quest.
Outland Instances (Both factions)
Hellfire Citadel (Peninsula)
This fortress is comprised of four instances. One raid instance, two low level and one level 70 instance;
The Ramparts (60+)
A fairly small instance, clearable within the hour by an experienced group. There are only three bosses, of which the second one is optional. You do need the second one for a quest in here, and most groups will kill all three for the loot anyway.
The Blood Furnace (61+)
Slightly bigger than The Ramparts, The Blood Furnace is also a little harder. There are again three bosses, of which the second one has a fairly long pre-boss event. At a certain point you find yourself in a large room full of caged Fel Orcs. You pull a lever and a cage opens, sending three fel orcs in. Once they die (or sooner if you are too slow to kill them) the second cage opens. There are six cages, and the later cages contain up to five orcs. They are not all elite, but the event can be quite a challenge.

The Shattered Halls (70)
This zone is quite difficult as far as the level 70 instances go, it contains multiple very large pulls of 5-6 elites at a time where good crowd control is a must.
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This is the home of Magetheridon, big fat Pit Lord.
Coilfang Reservoir
This is a large pump installation in the center of the big lake in Zangarmarsh. There are three 5-man instances and a raid instance.
The Slave Pens (62+)
This instance has three bosses and is reasonably easy to do. Many if the pulls in this zone can be skipped completely, which is worth considering if you have a good group. The area past the second boss is difficult because of the large pulls.
The Underbog (63+)
Four bosses in this one, no real places of extremely difficulty, although the area just beyond the first boss can be tough as the pulls are fairly big.
The Steamvault (70)
Fairly many multi-mob pulls in this one. Three bosses. Steamvault contains one of the two items required for the 'The Second and Third Key Fragments' quest required for the Karazhan key.
Serpentshrine Cavern (Raid)
Lady Vash's home
Auchindoun
Auchindoun is a large ruin in Terokkar forest in the center of the Bone wastes. It is split into four 5-man wings:
Mana-Tombs (63+)
Three bosses. Many mobs are skippable, but you may wish to kill them all regardless because of the escort quest taking you through the area later on. The adds could pprove problematic if you didnt clear everything.
Auchenai Crypts (64+)
Many mobs in this instance will spawn non elite adds when you engage them. Be mindful of this, it makes certain pulls challenging. Two bosses.

Sethekk Halls (65+)
Sethekk has alot of trash mobs for only two bosses, which can be a pain. Some pulls towards the end are challenging and require good CC to deal with successfully.
Shadow Labyrinth (70)
After the first boss, you will need to deal with large groups of elites. Good CC is instrumental to success here. More large pulls follow throughout the rest of the instance. The third and fourth boss can be quite difficult. You will find the First Key Fragment for Karazhan in a pot near the last boss. I do believe you can get it without killing the last boss, should he prove too difficult.
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Located in Tanaris, on Azeroth. The Caverns allow you to see some events that happened in the history of warcraft.
Old Hillsbrad (66+)
In this instance you set fire to the barracks in Durnholde and save Thrall in the commotion. Along the way you encounter three bosses. None are particularly tough, but the fact that this is in fact one huge escort quest can be a little unnerving. Luckily, should you wipe and Thrall die, you do not have to do the whole escort over, it will pick up at the last place you talked to thrall.
The Black Morass (68+)
Portals spawn in three fixed locations at random. Several mobs spawn quickly after another, which must be killed. When done, look for the next portal and commence killing. At set intervals a boss will spawn. Once the third and last boss is dead, the instance is finished.
Mount Hyjal (Raid)
The event taking place this time is Archimonde's defeat on Mount Hyjal.

Tempest Keep
This large structure hovers east of Netherstorm and is split into four wings. Three 5 mans and a raid instance.
The Botanica (70)
With a total of 5 bosses this makes for a rather rewarding instance.
The Mechanaar (70)
A fairly small zone, with alot of skippable groups. Three bosses and two minibosses that dont drop loot. They do drop badges on heroic mode though.
The Arcatraz (70)
The toughest of the three 5 man tempest keep zones, 4 bosses total. You will find a key fragment for the 'The Second and Third Fragments' Karazhan key quest here.
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Home to Kael'thas Sunstrider, leader of the Blood Elves on Outland.