Once you're back at the well known room, walk round it, take care of the archers and open two experience chests. You'll also be able to save your game.
The path then leads to a long and narrow corridor, in which you will meet a new enemy type - the Siren. It's the only path available, so you pretty much can't miss. Get rid of the enemies and run to the end.
The Sirens are quite nasty and can be quite a pain, especially if you're playing on higher difficulties. Their main and most deadly power is the ability to partly disappear - they change into a sort of a blue ghostly form. While they're in this form, you can't damage them. In order for them to return to their material form, you have to use the Head of Helios on them, then you'll be able to damage them.
They mainly focus on attacking at close range with their hands. However do have some aces up their sleeves: they can create a medium range sound wave and they can also stun you - once they do hit you, the screen will turn blue, you'll lose control over Kratos and will have to move the left stick quickly in order to regain control
In order to completely kill this enemy, you have to weaken him twice. The first time a QTE mark appears above its head, Kratos will only do a throw. The second QTE kills the Siren, which leaves some mana behind. It's also worth to notice that they both don't require any interaction.
At the other end of the corridor there are some round stairs and an experience chest by their base. Open it and enter the big room.
Once inside, head left first. There's a vent which will allow you to fly up.
There you will find two chests: the one on the left contains a Phoenix Feather [!!!], while the one on the right will give some experience. Absorb the red orbs and jump to the nearby metal platform (avoid falling down).
There are four movable drums. You have to turn them so that the picture below shows a sword. In order to do that (beginning from the left side) turn the first one once, leave the second untouched and turn the last two three times each.
Before you jump down, head to the platform on the right side [RS]. There's an experience chest there. Afterwards jump down.
Now you will have to demonstrate your dexterity. Approach the mechanism with the joypad buttons and begin the mini-game. The key is to press the buttons as they enter the marked spot. One mistake will reset the whole game and you'll have to start from the beginning.
If everything goes well, a lever will appear - pull the platform will start going down. During the ride you'll get attacked by shielded enemies. As you don't yet have the weapon needed to break through the armour, use the nearby Gorgon to change them into stone and then crush them.
After getting to the very bottom, leave the lift and once more enter the familiar room. This time stairs will appears. Take them upstairs.
Afterwards pull the lever by the Pandora's Box. This way you will unlock a nearby door and summon a few archers - kill them and go through the door.
Once again you'll find yourself in a long corridor. As you go deeper, two Wraiths will attack you. Get rid of them and go down the stairs to the very bottom (there's a hidden chest with a Gorgon Eye inside [!!!]).
Go straight and there will be a save point. Be sure to use it, as you're in for a tough fight in just a moment.
There's an open area in front of you. Before you go out, replenish your health and mana using the two nearby chests.