There are three things that you'll need to do in the Naga Temple:
a) Rescue Valdane. This involves finding a gold key and using it to unlock his cell. The gold key can be found in the prison on the eastern side.
b) Acquire the gas mask (#10). This is optional, but the gas mask prevents all damage from gas attacks, and so it is extremely useful.
c) Acquire the Relic of Power. For this you'll have to find a sigil (#14) and then use it to activate a teleporter (located on the eastern side). The teleporter will take you to a special part of the temple where you'll find the relic, and where you'll also have to kill the naga queen.
You'll probably get poisoned quite a bit in the temple. There are a couple fountains (#1) that can cure you of poison, but those fountains are located early in the temple, and so you'll have to rely on potions and spells (and perhaps the gas mask) later in the temple. In other words, it's a good idea to stock up on cure poison potions before going inside.
The Naga Temple is also called the Dungeon of the Moors.
Most of the treasure chests in this part of the temple were added in the Collector's Edition.
1 - Fountain
The fountain will heal you and cure poison on you.
2 - Locked Door
You can open the door using the lever on the western side.
Note: There are three guards in the temple that you can rescue. If you follow the guards all the way to the exit from the temple (Exit A), then you'll earn 10,000 experience each. The guards will all try to go through this door, so if you leave it closed, then they'll get stuck behind it, and you'll be able to ignore them when you see them and then escort them out when it's convenient for you.
3 - Colossus Trap
When you enter this room, the door to the south will close behind you, and a big snake called the Colossus of Myrza will attack you. The snake is reasonably nasty, but it only has a melee attack, and so you should be able to kill it, given a few healing potions or spells. Once the snake is dead, the door to the south will remain closed, but if you loop around to the prison (#4), you'll be able to pull a lever there to re-open it.
4 - Prison Area
You'll find two guards in the cells on the western side. You can free them by pulling the levers in the northeastern corner. If you follow the guards all the way to the exit, then you'll earn 10,000 experience for each one.
There is also a lever on the column on the eastern side. If you pull it, you'll open (or close) the southern door in the Colossus Trap room (#3).
5 - Closed Door
This door leads to a small maze where you can find the gas mask, but it will only open after you've rescued Valdane.
6 - Levers and Doors
The lever at #6a will open the door at #6b. The lever at #6c will open the door at #6d. The lever at #6e will open the door at #6f. You'll need to pull all of these levers to reach the chest with the gas mask (#10).
7 - Loose Bricks and Secret Doors
The loose brick at #7a will briefly open the secret door at #7b. The loose brick at #7c will briefly open the secret door at #7d. If you're playing a regular version of Dungeon Lords, then you won't find anything in the secret rooms that the loose bricks reveal. If you're playing the Collector's Edition, then you'll find high level chests and mimics inside.
8 - Battle
To get into the room here, you'll have to deal with a few naga firespitters and two fire gazers. This is a tough battle, just because all of the enemy fire spells will keep pushing you back out of melee range. Summon spells can be useful in the fight, and if you have a crystal ward of fire, now is definitely a good time to use it.
When fighting the battle, it's a good idea to retreat around the corner and try to fight the enemies a couple at a time. You might even be able to get them stuck in the mechanical traps in the hallway, and gain extra damage that way.
9 - Lever
Pulling the lever will open the door at #9a.
10 - Chest
You'll have to disarm a level 6 trap on the chest, but inside you'll find the gas mask.
11 - Mechanical Trap Room
There are numerous mechanical traps in this room, but they're not as nasty as they look. (You won't, for example, get decapitated by the rotating axes.) What you'll need to do here is dodge the traps as best you can and make your way to the alcoves to the north and east. In each alcove you'll find a lever, and pulling both levers will open the door to the south.
12 - Cell over Valdane
The grate in the floor here looks down into Valdane's cell, but there isn't any way to go down from here and rescue him. You'll have to find his cell door and the gold key that opens it in order to free him (they're both in the eastern half of the temple).
13 - Ghoul Trap Room
When you walk into the room, the doors to the north and south will close, and the four cells to the east and west will open, allowing several ghouls to attack you. To get out of the room, you'll need to press the button on the middle, western column. That will briefly open the secret room between the two cells to the east. Inside the secret room, you'll find a lever that will open the exit doors.
14 - Sigil Room
The eye of the sigil is located in the container in the center of the room. To open the container, you'll need to pull the lever on the southern wall. The sigil is protected by a noxious gas. If you have the gas mask (#10), then you can jump into the container, take the sigil, and jump back out without taking damage. If you don't have the gas mask, then you can still try to jump in and grab the sigil before the gas kills you... but the gas will probably kill you.
Note: In MMXII, the noxious gas will poison you, but it won't do much damage, and so the gas mask isn't really needed.
You'll need the eye of the sigil to enter the small area where the naga queen and the Relic of Power are located (in the eastern part of the temple).
15 - Treasure Chest / Mimic (Collector's Edition)
If you're playing the regular version of Dungeon Lords, then the chest here is the only one you'll find in this half of the temple. If you're playing the Collector's Edition, then you'll find a mimic here instead.