This level is rather balanced when it comes to the battle - puzzle ratio, which doesn't necessarily mean it's easy. New enemies who are sure to cause you some problems and test the skills that you have gained until now will appear. Apart from that, the scenery changes to a bit more civilized one.
You begin the adventure in a corridor with the first puzzle waiting for you. The eastern branch is a dead end, filled with debris.
Visit this cell first and you will find two more Quarrels and the Scroll of Fire Shield.
Inside this seemingly empty room, find a button on the wall to open a passage to the adjoining cell. Inside of it, on a shelf, you will find a key to the door in point (5) and a Box. The Box contains some food and alchemic herbs and what's most important - the Scroll of Light.
Inside the passage between the cells you should find a hardly visible button on the wall, which can open a secret room with the Scaled Cloak.
In order to open this door, use the Iron Key which you found in the cell.
Inside this room you should find one of the two dragon statues. Their setup isn't random, but we will get back to that in point (15). Take two Breads from the shelves here, as well as a Shuriken from the corridor in the south. The corridor leading west is collapsed, so you will need a key to venture deeper.
Inside this room you will have to solve a puzzle resembling one of the challenges on the previous level. Take a look around the walls and you will see three metal buttons - two on your side of the pit and one on the opposite. In order to safely reach the alcove on the other side, you will have to press the buttons in the proper order, so that the trap doors create a bridge.
I have marked the buttons on your side with 1 and 2 and the opposite button with 3. The proper order in which you need to press them is 1 > 3 > 1 > 2 > 3. Pressing the last button will make going back impossible, though you will reach the alcove with the Iron Key. Unfortunately, as only you take it, two small rooms will open above the trap doors, out of which a Crowern (piece of cake) and a Wyvern, which is a far difficult variety of the flying reptiles, will jump out. The situation is made worse by the fact that you have a very limited space to manoeuvre. You need to lure the Wyvern as close to you as possible, or else it will attack you from afar and you won't even be able to reach it.
After defeating both reptiles, press button 3 once again and return to the northern part of the trap door. Take the herbs from the room beside the buttons and press 1 to close the trap door above the second opened room, where you can find Toorum's Note regarding a secret which can be found in the next room (9).
Before returning to the corridors however, jump down into one of the trap doors. Here you can find a secret, though take a rest and regenerate your strength, as you will have to fight the same set of reptiles as before. Luckily here you have much more space to avoid the Wyvern's attacks. Once you're done sleeping, find the button hidden on the right wall beside the portal. Behind you a room with the enemies as well as a secret will open. Inside you can find the Lightning and Fire Bombs.
Inside the room beside the trap doors puzzle you have probably noticed a lever and two buttons - one beside the grate behind which the puzzle was and one by the wooden door. In order to open the wooden door, you will need to proceed as follows: pull the lever down, press the button by the grate, pull the lever up and down several times (pulling it up in the end) and press the button by the wooden door. Two Herders - Small and Adult, so both highly toxic - will come out. Inside the secret room there's a Conjurer's Hat for you mage and some food.
The Iron Key from point (7) opens this door. Be ready to fight Crowerns and Wyverns at once. Don't worry if part of your team dies, as there's a Crystal of Life right beside.
Right behind the door you will find a button hidden on the wall. Pressing it will grant you access to the alcove, inside which you can find a great weapon for your Rogue: the Fist Dagger. Note the herbs growing right beside the hidden button.
Crystal of Life. Jump into the trap door beside it to find another Toorum's Note.
After you enter this corridor, you're in for your first, rather unpleasant meeting with the Giant Crab. Luckily the Crystal of Life is nearby. After the fight, take a look inside the northern alcove to find the Ring Greaves between some bones.
A descent to the next level can be found here, though it's locked by two gates for which you need two Ornate Keys. You can gain one of them by heading east and the other one in the west. Behind the barrels by the door to the side corridors you can find a Toorum's Note, enigmatically explaining how to open the Iron Door in this room.
In order to reach the content guarded by the Iron Door, you need to take a rest in the spot marked on the map, beside point (13). At this point the sights of two dragon statues cross (you will come across the second one at point (19)). After a short nap, the door will open and one the other side will find the Lurker Vest.
Head to the eastern corridors. Here, a timed puzzle awaits you. Prepare a rock or another throwing weapon and pull the lever by the grate. At the end of the western corridor there's a pressure plate - quickly throw a rock on it and run towards the newly opened door in the east. Don't worry if the alcove beside the pressure plate doesn't appear on your map, as it opens only after a few failed attempts of solving this puzzle. Inside you will find only a hint.
As the way back to the main corridor is temporarily cut off, this room will be the safest place to stay. Collect the resources found inside and note that you can lock yourself inside and therefore safely rest.
These corridors will at first look completely differently on your map. Further elements will successively open as you travel through them. However be very careful while doing so. Each time you hear the sound of a wall opening, retreat closer to point (17) where you have more space to fight and eventually rest. Each opening tunnel will let out a monster, e.g. a Giant Crabs or Adult Herders. Lock yourself inside the cell (17) after each fight and regenerate before returning to the tunnels. Once all the tunnels are opened, collect the herbs growing in the nooks, Fire Bombs (right beside (17)) and the Warhammer found on the shelf in the marked spot. After searching the corridors, return to point (19).
Inside the room with the dragon statue there's the first Ornate Key. In order to return from there to the main part of the underground, pull the lever by the wall, quickly run into the familiar corridor (16) and pull the lever found there.
After you pull the lever at (16), immediately return to the room with the dragon. If you make it in time before the ticking stops, you will find an open cage and inside a secret with some Arrows and the Huntsman Cloak.
In the western part of the corridors you're in for a task requiring reflex and a bit of good memory. The button on the wall on your right launches a series of trap doors, which eventually close and open again after some time. Beside there's also a lever which reverts their movement. Switch the lever and start the mechanism.
Start off by taking a closer look at the track which is created by the trap doors closing (marked on the mini-map) and afterwards head along this moving "bridge".
On your way to the other side of the trap doors you might have noticed a rather well visible button on the wall - if you press it while walking along the trap doors, you might be able to reach a small shelf with the Brace of Fortitude. Grab it quickly before you fall through the trap door which will open soon. On the lower floor you will have to fight your way through a couple types of Skeletons. Search the room after the fight to find some herbs and food.
On your way up the stairs (from the room with the trap doors) you will find a Boiled Crag Beetle and another Bow. Pull the lever to return to point (21).
Here you will find another button and a lever controlling the trap doors. You will need them on your way back from (25).
Start off by defeating the Giant Crab here and rest. The chamber seems empty, but only seemingly. Behind the pillar with statues you will find the second Ornate Key. If you have ever seen an Indiana Jones movie, you will probably guess what will happen after you take the key - the room will be filled by enemies coming from the newly opened corridors. You can avoid this as cunningly as Indiana would, putting an item on the shelf before taking the key. However it's probably not worth it, as by fighting here you will gain quite a lot of experience and additionally find some herbs and Lightning Bombs in the open alcoves. Return to point (14). On your way you are sure to note that various groups of Skeletons wander the corridors - kill them all for additional experience and use both keys to reach the lower level.
Stairs to level 4.
Stairs to level 6.
Stairs to the room above the trap doors (level 6).