Head up the stairs outside towards the arena to a new enemy.
++ NEW ENEMY: CERBERUS++
For this battle, I suggest activating Rage and unleashing your Fury on the three-headed dog or use your maximum Poseidon's Rage if you have it . Kill it and its little mini-hounds to unseal the red door. Kill the big dogs and the mini-hounds ASAP. They will grow into bigger hounds if you let them be!
Continue through the mountain and kill all the enemies that emerge (Mega Cyclops, archers, mini-hounds). Kill them all to unseal the door to the building at the other end.
Enter the room and pull the lever. As the room rotates, it reveals Blood Chests. To open them, wait and be ready in front of the gate. As soon as they open, head in and open the chests. You will only have a few seconds before the spikes come up! Wait for the room to rotate 180 degrees and it'll open a new doorway.
Head outside to encounter a new enemy.
++ENEMY: MEGA SOLDIER++
These guys are practically taller versions of themselves and wield a large sword. Use blocks and hard Combos to whittle their health down. Use the Circle Button when it appears to perform a fatality.
Head near the ledge and jump towards the mountain. Jump your way to the top and shimmy to the right. Open the Chest at the end to obtain another Muse's Key. DO NOT JUMP DOWN! Instead shimmy all the way to the left of the mountain to the other side and jump down. Kill the Mini-taurs and continue on.
SAVE GAME
==SACRIFICE TO THE GODS PUZZLE==
The turnstile is missing its handle. Head for the room at the other end and find it there. Go back to the turnstile and use it.
After the cutscene, push the cage down. Destroy all enemies that appear first before pushing the cage up the slope. Once that's done push the cage into the other room and onto the button. Turn the lever to fry the victim inside and open a new pathway.
==AREA: TRIDENT ROOM==
Head down the path towards the walkway to Poseidon's Trident. Remove the Trident from the pedestal to obtain the ability to breathe underwater indefinitely.
There is also a Chest with a Phoenix Feather in this room.
SAVE GAME
==AREA: UNDERWATER PUZZLE==
Jump in the water on the right side and dive into the large hole. Swim your way to the other side and jump out.Kill the mini-hounds and then open the door at the other end and pull the lever. Dive back in the water back to the Trident Room. Emerge from the water and destroy all the enemies that appear here.
Jump up the newly revealed pedestal to a circular pathway. Head down the pathway to a watery area. Dive down and swim to the other end.
==SUB-OBJECTIVE: KISS POSEIDON'S DAUGHTERS==
You'll encounter Nyad, a child of Poseidon here. Press the Circle Button to give her a Kiss. You'll obtain some goodies everytime you kiss her. There's four of these girls, three of them are hidden behind cracked walls in the water. Think you can find all of them?
Dive further down underneath her. Turn right and swim down a corridor with tons of spikes and chainsaws. If you don't get to the other side before the gaps close, you're dead. Use the R1 Button Dash to quickly swim faster towards the other side.
Emerge out of the water on the other side and kill all the enemies that appear in the dry area. Once that's done, pull the lever. Now swim back to the spikes and chainsaw corridor (use the R1 Dash!). Before heading to the opened Poseidon doorway, swim further and you'll emerge to a hallway with Mega Soldiers.
Kill them all. See that row of pots? Smash them all to reveal a chest with a Muse Key (if you have obtained TWO Muse Keys by now, this will be a chest with Blood Orbs). Now head back to the Poseidon door and swim further down.
SAVE GAME
==AREA: FLOODED PASSAGE==
There are a couple of breakable walls near the Save Point with hidden chests (if you want to find them).
See Poseidon's Statue? Dive underneath that to a mini-puzzle. You have to swim really fast to avoid being shoved by the barricades. There are two safe places to stay safe. Use them and use a fully charged R1 Dash to quickly make the most of this situation. You need to dive under the cage at the end of the passage. This will take a lot of tries to get right.
Once you make it, emerge out of the water and onto dry area. Kill the Mini-taurs first.
SAVE GAME
==AREA: AMPHITRITE PUZZLE ROOM==
There's a puzzle to be solved here but how? How do you find your way out of the room?
Simple. When you pull the lever to raise the pedestals, jump into the water and dive quickly to the bottom pedestal at the very end and you'll see a small room under one of the pedestals. Enter that room to exit this area. Swim out to the next area. You might also want to get the chests near the statues first before you proceed.
Welcome back to the Rings of Pandora.
SAVE GAME
==OBJECTIVE: USE THE MUSE KEYS==
If you have obtained both of the Muse Keys, go back to the turnstile at the Rings of Pandora and turn it until you see a doorway. Enter that doorway and insert both Keys to open a special "Chamber of the Gods". I'll let you see it for yourself
Before you go to the next area, turn the turnstile until you get the Save Room again. This will make your life easier at a later stage.
Exit back to the Steamroller Room (while killing fools on the way) and jump into the large pool of water. Look for a hole to dive into and swim all the way to the other end. Emerge out and head down the hallway.
==AREA: MINOTAUR ROOM==
Ignore the lever at the top for now. Look for a crack on the wall near the stairs and break it open.