"I can't STAND Tulla...everyone is so bloody boring. You'd think it was a damned graveyard with all of the walking dead around here..."
You can find a lot of good magical loot in the city, but if you attack any magicians to get it, the entire city will turn again you. Also, if you joined the dark elves in T'sen-Ang, the gate guard won't let you in, and fighting him will turn the city against you, giving you little reason to go in.
1 - Entrance Gate 
The only way into Tulla is to use the teleportation gate outside the city. When you talk to the elf next to it, he'll inform you that Jorian is expecting you, and he'll allow you to use the gate.
2 - Study Room 
Inside you'll meet Herzod Munk, who is something less than an eager student. He'll ask you to take his final Phantasm College exam for him. If you agree, he'll give you Fa'al Kin's Coin, the key to the exam area. To take the exam, just enter the arches near the building (#3) while the coin is in your inventory. You'll be teleported to Fa'al Kin's Maze.
The maze is made up of several rooms. In the first room you'll find a pair of chests containing Fa'al Kin's equipment: amulet, boots, dagger, gauntlets, helmet and ring. To make your way through the "maze" you'll have to wear a particular piece of Fa'al Kin's equipment (or none of it) and then touch the teleportation stone in the center of the room. It doesn't matter if you wear your regular equipment or not.
The order of equipment is: no equipment, ring, amulet, helmet, boots, gauntlets, dagger, all equipment, no equipment. The last room also contains Fa'al Kin's Chalice, which is what you need to return to Munk.
When you give the chalice to Munk you'll receive 8500 experience and a scroll of phantasmal fiend. After completing the quest you might want to ditch Fa'al Kin's Coin somewhere so you don't accidentally enter the maze again.
3 - Teleportation Arches 
You'll only be teleported if you have Fa'al Kin's Coin (see #2).
4 - Canto Building (cross-circle) 
The canto buildings are involved in the quest to speak with the ghost of Pelojian (pertinent information can be found in the Tulla Interior, #1 and #4). Each building contains a canto, and the four cantos give clues for how to step on the pressure plates surrounding Pelojian's Pool (#5).
However, there isn't anything special about the locks on the buildings. If you can pick locks or if you have the unlocking cantrip, then you can get into the buildings with no problem. Otherwise, you'll have to go through a fairly tedious process of stepping on the pressure plates scattered around Tulla.
The pressure plates have the symbols of the magic colleges on them. If you don't know the symbols of the magic colleges, bring up your character page and then click on the "magic" button. The symbols for all 16 colleges are shown there.
To open this building, you must step on the pressure plates for the earth (#4a), air (#4b), water (#4c), and then fire (#4d) magic colleges. Once you do so you'll find "canto1" which reads, in part, "the final step before you come to me." You should also find Pelojian's Amulet.
The amulet is the only thing directly needed to summon Pelojian, so the other buildings may be skipped if you wish.
5 - Canto Building (spiral) 
See #4 for background information.
To open this building, you must step on the pressure plates for the conveyance (#5a), force (#5b), morph (#5c), and then phantasm (#5d) magic colleges. Once you do so you'll find "canto3" which reads, in part, "not first above but just underneath."
6 - Canto Building (stick figure) 
See #4 for background information.
To open this building, you must step on the pressure plates for the necromantic white (#6a), necromantic black (#6b), summoning (#6c), and then nature (#6d) magic colleges. Once you do so you'll find "canto4" which reads, in part, "the first to the last."
7 - Canto Building (star) 
See #4 for background information.
To open this building, you must step on the pressure plates for the divination (#7a), meta (#7b), mental (#7c), and then temporal (#7d) magic colleges. Once you do so you'll find "canto2" which reads, in part, "one step which begins a journey."
8 - Pelojian's Pool 
As long as you're wearing Pelojian's Amulet (see #4), you can activate the pressure plates surrounding Pelojian's Pool. The correct order for stepping on the plates is described by the cantos found in the canto buildings (#4-7) -- star, spiral, stick figure, cross-circle -- followed by the symbol for magic. Once you activate the symbols, Pelojian will appear. You don't have to wait for midnight.
Pelojian will tell you about Vendigroth and the Device, and he'll mark the location of Vendigroth on your world map.
A. Entrance to Tulla Interior.
1 - Mural of Enlightenment
The mural is the key to figuring out how to speak with the ghost of Pelojian (see #4). The mural depicts four large symbols, each surrounded by four small symbols. The 16 small symbols represent the 16 colleges of magic, and they can also be found on the 16 pressure plates outside. The four large symbols have an unknown meaning (the upper-right is surrounded by Nature, White & Black Necromancy and Summoning so it could stand for "life", while upper-left is surrounded by Fire, Water, Earth and Air so it could stand for "elemental"), but they can be found on the four canto buildings outside.
If you can't pick locks or cast the unlocking cantrip, then to open each canto building you must step on the four pressure plates associated with it, and do so in the correct order. Start with the symbol at the 12:00 position and then work clockwise. The last symbol should always be right in front of the building you want to open.
2 - Teleportation Circle
You'll be able to use the circle if you attempt to become a master of a college of magic (see #5a-o).
3 - Jorian
Before Jorian will let you in to see Simeon Tor (#4), you'll have to go back downstairs and look at the mural (#1), even if you looked at it before. Then you'll be able to open the doorway to Simeon Tor's room (Exit C).
4 - Simeon Tor
When you tell Simeon Tor that Nasrudin sent you to discover the location of Vendigroth, he'll reply that Pelojian is the only one who can help you. Then he'll add that Pelojian has been dead for over a thousand years, but that he appears nightly at the pool in the center of the city (Tulla Exterior, #8). Tor won't have any idea about how you might get the spirit to speak, but he'll hint that anything Jorian (#3) has had you do is probably important.
5 - Master Mages
The mages can train you to become a master of a magical college. They'll give you a mastery medallion, and the medallion will allow you to use the teleportation circle in the center of the level (#2). The circle will transport you to a series of rooms that have puzzles and / or enemies in them. If you can complete the puzzles and kill the enemies, usually by using spells from the magic college, and return to the master mage, then you'll be able to cast the spells in the college at half the fatigue cost (so it's useful to become master of, say, the force college). You can only become a master of a single college; other masters will give you their medallions, but stepping on the circle while already a master of a certain college does nothing. You don't need to wear the mastery medallion for the reduction in fatigue cost.
A few in-depth notes... You can receive the master medallion, finish the associated dungeon, and receive the associated reputation even if you do not have all five tiers of spells learned in a specific college (even with no tiers learned), but you will not receive any benefits in that case. You must have all five tiers of spells learned in a college to master it. Even if you receive mastery and then unlock the tiers you have left afterwards, the mastery benefits will not work. Still, it can be a good idea to take a college mastery quest even if you do not want its benefits: killing all the monsters in some of the dungeons (e.g. in the Temporal dungeon) can net high amounts of experience. Regarding the benefits: it seems that sustainment costs are affected as well as the initial casting costs. Although mastery does not directly reduce the cost of spell sustainment, it seems to double the time it takes for a sustained spell to take away from the player character's fatigue.
The masters are listed below:
a) Liam Raymond, Master of Meta
For the mastery quest, in the first area you'll find three lords of the damned, and one of them will have a key allowing you to unlock a door and go through a portal. In the second area you'll face a single elf wizard. The wizard will also have a key on his body, and it will allow you to access another portal past another locked door. This last portal will take you back to the main level of the Tulla Interior.
b) S'Btin'ka, Mistress of Morph
If you agreed to help Cynthia Wit in Caladon, then you'll be able to buy lycanthrope restorative from S'Bin'ka for 2000 (or 1000) gold. You can also just steal it from her.
For the mastery quest, in the first area you'll find a very small door. You'll have to cast the shrink spell on yourself to pass through it. In the second area you'll face three skullcrushers. One of them will have a key. The key unlocks the door leading to a portal, and the portal will take you back to the main level of the Tulla Interior.
c) Ferko Lydell, Master of Nature
For the mastery quest, in the first area you'll find a whole slew of level 30 firejumpers. You're probably supposed to use the empathy spell to tame the firejumpers so you can get to the chest in the center of the area, but that's never worked well for me, and the only way through the firejumpers seems to be to take them on in combat (easier said than done, but worth a lot of experience). However you get past the firejumpers, you'll find a key in the chest, and the key will allow you to move on to the second area.
In the second area you'll find a bunch of brute fangs, but this battle is much easier because there are fewer brute fangs to worry about. Once past the brute fangs you'll be teleported back to the main level of the Tulla Interior.
d) D'Ary, Master of Necromantic Black
For the mastery quest, in the first area you'll find a bunch of level 3 lesser krags guarding a chest. Once you defeat the krags and open the chest, you'll find a key allowing you access to the second area. In the second area you'll find a pair of frigidons, one of which will be carrying a key. The key unlocks the nearby door, allowing you to leave the area and return to the main level of the Tulla Interior.
e) Bilko Gavin, Master of Necromantic White
For the mastery quest, in the first area you'll find some dragarons fighting skullcrushers. Any dragarons that survive will attack you, so you can either heal the skullcrushers so they fight longer, or just wade in and attack the dragarons, or ignore the battles completely since the door on the other side of the area is unlocked. In the second area you'll find a bunch of molten arachnids to kill, but eventually the hallways will lead to a portal, and the portal will take you back to the main level of the Tulla Interior.
f) Daemon Gardi, Master of Phantasm
For the mastery quest, in the first (and only) area you'll find some frigidons and dragarons to kill. Past them, through an unlocked door, a portal will return you to the main level of the Tulla Interior.
g) Harold Sumner (aka Tom Smith), Master of Summoning
For the mastery quest, in the first area you'll face a bunch of krag warriors and a single skullcrusher. The skullcrusher has the key allowing you to return to the main level of the Tulla Interior.
h) S'yala, Mistress of Temporal
For the mastery quest, in the first area you'll face three level 25 wargunns. Then in the second area you'll face over a dozen level 25 wargunns. The portal in the second area leads back to the main level of the Tulla Interior. None of the doors in the two areas are locked.
i) Ve'Tura, Mistress of Conveyance
For the mastery quest, you'll first find yourself on a platform. To get to the portal on the other side, you'll have to use the spatial distortion spell. (If you try to walk, you'll just get teleported back.) In the second area, you'll find a rock sitting on a portal with eight pillars of different colors surrounding it. The pillars farthest from the rock can't be moved, but the four closest to the rock can be moved using the unseen force spell. The idea is to push the four movable pillars to the corners of the area so that the unmovable pillar between them is the combination of their colors. That is, the pillars should go, in clockwise order: orange, red, magenta, dark blue, light blue, green, lime, yellow. If you get stuck or want to avoid using Unseen Force, you can also move the pillars by holding the ALT key then clicking on them when they are next to you. Once you get the pillars in the correct order, the rock will disappear and you'll be able to return to the main level of the Tulla Interior.
j) Cassandra Johanson, Mistress of Divination
For the mastery quest, you'll find yourself in an area with six teleportation circles. If you cast the sense hidden spell, you'll discover that five of the circles are guarded by level 30 invisible crushers, and one isn't. The unprotected circle will take you back to the main level of the Tulla Interior. The other five circles won't do anything.
k) Wel' K'ene, Master of Air, and Addo Terrin, Master of Earth
For the air mastery quest, in the first area you'll find some tattered bowmen and numerous traps. Potentially you could cast body of air and avoid all the traps, but that won't help your followers any. Plus, the traps aren't very powerful, so the easiest thing to do is just walk through the room and take your lumps. In the second area you'll find six more tattered bowmen and a locked door, but I have no idea how you're supposed to unlock the door, other than by picking it or using the unlocking cantrip. Maybe a key's supposed to appear on the skeletons but doesn't? Past the locked door you'll find the portal back to the main level of the Tulla Interior.
For the earth mastery quest, in the first area you'll find two lines of tattered bowmen, which should be easy to defeat. In the second area you'll find some skullcrushers and a 3x3 configuration of chests. The key to the next area is in the chests. In the third area you'll find some widower spiders. No key is required to exit the area and return to the main level of the Tulla Interior.
l) Naph'Tha, Mistress of Fire
For the mastery quest, in the first area you'll find a walkway of flames. You can cast body of fire on yourself to avoid most of the damage, but that won't help your followers any. So either tell them to wait before starting the quest, or else strip them first to preserve their armor. In the second area, you'll have to fight a few water elementals. Beyond the water elementals you'll find the portal leading back to the main level of the Tulla Interior.
m) V'ed Eckes, Master of Water
Eckes will tell you about a student he sent out to find a gem of water purity, and how even though the student found the gem, he is now considered tainted and can't be allowed back inside the walls of Tulla. Further, he'll ask you to go outside to collect the gem from the student. If you're nice or persuasive, you might be able to convince the student he doesn't really belong in Tulla, and he'll give up the gem peaceably before he leaves. Otherwise you'll have to rob or kill the student to get the gem. Since the student has a Clarington rifle and machined plate, as well as the gem, you might want to kill him regardless. When you return the gem to Eckes you'll receive a robe of fire protection and 3100 experience.
Note: The gem quest is a reference to Fallout.
For the mastery quest, in the first area you'll face a bunch of kite berserkers, and then in the second area you'll face a bunch of monkey swarmers. None of the doors are locked, so you can just run through the fights if you want.
n) Rys'Ard, Master of Force
For the mastery quest, in the first area you'll find two frigidons and a locked door. To get past the locked door you can just disintegrate it (or use the unlocking cantrip, or whatever). In the second area you'll find a bunch of monkey swarmers. In the third area you'll encounter a level 30 siren queen. Since she isn't carrying anything, it's safe to use disintegrate on her. Past the queen is the portal back to the main level of the Tulla Interior.
o) Edgar Jerryl, Master of Mental
For the mastery quest, in the first area you'll face a pair of wargunns. If there's a clever way to unlock the door beyond them, I don't know what it is. (Perhaps you can dominate the wargunns and order them to break down the door, but you probably have the unlocking cantrip, which is easier to use.) In the second area you'll find some chests guarded by frigidons. Each of the chests contains a key, but you need to find the "tulla-battle-mental" key to leave the area and return to the main level of the Tulla Interior.
A. Exit back to the Tulla Exterior.
B. Stairs between the main level and the upstairs landing.
C. Doorway between Simeon Tor's room and the upstairs landing.
D. Teleporter to the Ring of Brodgar.
E. Teleporter to Caladon.
F. Teleporter to Black Root.
G. Teleporter to Tarant.
H. Teleporter to Ashbury.
I. Teleporter to Stillwater.
Bugs and exploits
- It is possible to enter Tulla even if the guard doesn't allow you. To do so, you need to set your waypoint on the World Map to travel over Tulla, but not directly on it. It is advisable to save before you attempt this since it's possible to get stuck in the city walls. While travelling above the location on the World Map, quickly press the button to stop travelling (the large waypoint follow button on the right) and after a few tries you should end up inside. Note that apart from getting stuck in the walls you may also end up inside one of the locked houses, so have lockpick skill or Unlocking Cantrip spell ready. This trick may also have uses elsewhere...
- Got stuck in one of the Master Mage training locations? No problem. You should be able to use the World Map to travel outside and attempt it again. (not tested)