Back in Arcs
You'll be back at the inn with the rest of the adventurers who went to the Tower of Light. The main thing of importance is the poster on the wall. They want people to explore the Ancient Ruins and find something. Yup, that's your next destination but you won't find a road that leads there like the last one. Instead you'll have to do some "underground" investigation to find the way.
Go see the blacksmith on the far left side of Arcs and talk to him. He'll tell you that the price of Dragon Scales has dropped considerably since the other adventurers came back to town and sold lots to him. He'll offer you 2000 gold for each Dragon Scale you have. This is the best price you'll get so you might as well sell them now. After you do he'll tell you about the tunnel he was digging in his spare time that leads to the Ancient Ruins. You can't reach the Ancient Ruins yet since he already mentioned the rocks that are blocking the path.
However he'll also mention that an Axe may destroy the rocks blocking the way. So, go find an Axe. Hint: you'll find one in the basement of the tool shop, where the guy was selling weapons he didn't have in the first place. Note that you can't go to Barn the weapons dealer until you talk to the blacksmith. He won't have anything on sale until you do. So, once the conversation with Marlon is over, go over to the tools shop, then head downstairs and buy the Battle axe from Barn.
You may buy the Boomerang because it'll ease your life a lot, even though you'll find it for free later. Head back upstairs and talk to the Tools shop guy to stock up on items for the journey to the Ancient Ruins. Let's see, you'll need a good few of the following: Cape Herb and Antidote Herb. After that's done, go check out the Magic Shop. There's some interesting magic here, so buy as many spells as you can.
Katabanch & Vince
Talk to the people at the bar and at the smith's: their sons Katabanch and Vince disappeared. They can be found in the cave to the Abell ruins, in one of the small rooms. Unfortunately, completion of this side-quest does not grant any prize (as it was in The 7th Saga, the previous title in the series).
Cave to the ruins
Once that's done you're ready to go! Return to the blacksmith's and into the tunnel he carved out. Once there use the Axe on the rocks to smash them so you can continue on.
The caves beyond the tunnel are, unfortunately, non-linear so hug one wall until you get either to the exit or back to the entrance. In the large cave there are many doors that lead to small rooms. Only one room has a second exit, leading to the ruins. In the dead-end rooms you can find several useful items.
The trolls here are easy to kill but watch out for the Scorpions, they hit pretty hard and they'll poison you to boot!
The Abell Ruins
The subsections below give directions to find the keys in the titles.
First floor
Pazun, fairy of the Light Jade 
Enter the Abell ruins and save your game. Walk to the left and enter the second door. In this room you can drink at a restoring spring. Now, go back in the corridor and enter the room to the right. The chests contains the Light Jade, and the jade contains the fairy Pazun.
Cave room key
Leave the jade room and enter the next door to the right. You'll meet two enemy tetsujins, robots that also appear in The 7th Saga and Mystic Ark. The next room is dark and inhabited by blue bats, weak enemies that you already met on the way to the Tower of Light. One of them always drops a blue experience sphere: exit and re-enter this room repeatedly to quickly level-up Pazun or any other fairy. The next room holds the Cave room key. The only way to get to the chest is to hit it with some magic, until the floor appears.
- Cave room key: Found on the 1st floor of site of civilization. Point of the key shines to the southwest.
Dark room key
Following the hint on the key, return to the main corridor and use the key on the south-western locked door. Inside, after a corridor infested by enemies, is the Dark room key.
- Dark room key: Found on the 1st floor of site of civilization. Blackened brass key.'
Return to the restoration room, then follow the conveyor belt clockwise. In the nort-western corner of this floor there are three unlocked doors. In the first of the three you can find a heart and in the second one a drop of protect. In the third room you can meet Barness and Kashian, but DO NOT eat the roasted scorpion they offer you, or you'll get poisoned. Follow the conveyor belt eastwards and use the dark room key on the next door.
Underground key
The first room past the door is filled with tetsujins. The second room hosts a riddle, but the solution is on the wall. In the third room you have to face a magician, but upon defeat you can open the chest and get the Underground key.
- Underground key: Found on the 1st floor of site of civilization. Tip of key shines to the east.
There's just one locked door left on this floor, and it is in the north-east corner of the conveyor belt corridor. Past that door are the stairs to the second floor.
Second floor
Bottomless key
On top of the stairs is a save statue, but also Barness, who now acts as merchant and restoration source. Leave the room and follow the conveyor belt to the left (west), then enter the first door you see, before turning south. Here's a double puzzle. Five rocks are on a line of four, and the fifth is behind one of them. Push the second rock from the right twice, then push the back-row rock sideways. Now push the second rock from the left all the way south and east, until you put in on a switch. Walk back and repeat, pushing the rightmost rock first northwards a bit, then west, south and east until you place it on the second switch. In the next room is the Bottomless key.
- Bottomless key: Found on the 2nd floor of site of civilization. Tip of key shines to the south-southeast.
On the way back, use four rocks to open the other door. Use all the rocks exept the one on the left wall. A chest in the room contains the Banded mail. Follow the main corridor south and east. There's an unlocked room. The room behind it contains the Morning star. Ignore the next unlocked door for the moment and use the bottomless key on the following one.
Distance key
At the end of a long corridor full of bottomless pits (hence the name of the key), you can find a room with the Distance Key. That's not all: push the wall to reveal a secret passage. In the room below is a Source of Power.
- Distance key: Found on the 2nd floor of site of civilization. Tip of key shines to the south-southwest.
Remember the unlocked door we skipped in the main corridor? Now it's time to walk past it. In the next room there are two doors: one is locked (by the distance key), the other one requires the solution of a puzzle. The first hint says that there are three numbers. The second hint says that the product is 30: from elementary school maths, 30 = 2 x 3 x 5. The third hint is unnecessary, as we know already the numbers. The last hint says the order you have to press the switches. In the next room there is another riddle. It's quite cryptic. Here's the solution: the three numbers refer to months, weeks and days in one year. Past that door you can find a Boomerang for free: now you can sell the bow. Backtrack and use the distance key on the locked door.
Basement key
The next room hosts an easier puzzle, requiring timing rather than reasoning: push the three bouncing spheres so that they are parallel to each other and roll on the switches at the same. Past the door, follow the wininding corridor to a puzzle involving a red ball and three rocks. First, push the bottom rock twice and let the red ball follow you to the corner on the left. Second, pushe the other two rocks on the left and central switches. Third, call back the red ball, so that the lower rock will stop it, then it will go on the last switch. You can find the Basement key in the room.
- Basement key: Found on the 2nd floor of site of civilization. Tip of key point to the ceiling.
Backtrack and use it on the locked door. In the next room you can meet a merchant, then climb the stairs to the third floor.
Third floor
Oblivion key
Walk north to a save statue. Walk around this floor past several locked doors. On a platform you can find a chest containing a heart. Eventually, you'll get to a plaque with a hint: "the puzzle is hidden in the pillar". Push the left pillar to the right until it activates the switch. In the room you can find the Oblivion key.
- Oblivion key: Found on the 3rd floor of site of civilization. The tip of this key shines close-by.
Exit the door nearby: on the other side is locked! Use the oblivion key to open a shortcut to the save statue.
Flyer's key
After saving at the statue, leave the room of the oblivion key from the other door and navigate through a series of moving platforms. In the room at the end you can find the Flyer's key.
- Flyer's key: Found on the 3rd floor of site of civilization. This key is decorated with a strange metal.
This is the key that opens the door next to the save statue.
Invisible key
- Invisible key: Found on the 3rd floor of site of civilization. You will use this just ahead.
Use the invisible key to open the door in the north-west corner of this room. The Steel Sky key will be used later, falling from the 4th floor. About falling: a sign next to the Invisible key door says "walk five steps back". Do as it says, walking into the pit, and you can get to a secret room on the previous floor. The sign of the Eyesight Experiment Lab suggests to use the X-ray glasses to see the path through an apparently void chasm (reminds Indiana Jones...). Climb the stairs to the next room and save.
Fourth floor
Power room key
Outside the next door are large chasms and a floating platform that you can drive.
Drive the flyer hugging the wall to the left and up. In the corner, jump into the void to get to a secret room. Climb the stairs and repeat, this time jumping in a corner slightly to the left. In one of the two secret rooms you can find the Bone mail.
Use the X-ray glasses to see where is the door to the nearest room. In that labirynthine room the cape can be found: it allows to run as fast as the red balls. At the end of this maze you can find the Power room key.
- Power room key: Found on the 4th floor of site of civilization. The tip of this key shines to the south-southwest.
Laboratory key
At the end of the southwest set of room is the next key, the Laboratory key.
- Laboratory key: Found on the 4th floor of site of civilization. The tip of this key shines to the center of the floor.
You cannot use it yet, so let's think about a previous key:
- Steel Sky key: Found on the 3rd floor of site of civilization. A weapon is drawn on it. Use on the 3rd floor.
Go back to the central chasm and hug the wall to the left. A sign says: "The jewel rests below". Jump down to get a couple valuables, then leave the room and walk right to a locked door. Use the Steel Sky key to enter room and obtain the Steel buster sword. It's not all: follow the platform west until you reach the disappearing floor. Notice that there is an empty square tile between the solid and the disappearing floor: step in it to get to another secret room, where you can get the Cross helm. Now walk all the way back upstairs...
Control key
Well then, now the nearest unlocked door is to the south-east. There is a puzzle similar to a previous one: two balls (instead of three) and two rocks. Use the two rocks to prevent the balls from hitting the side walls. Push the balls to the door and to the central switch. Finally, push the two rocks to the other two switches. In the next room are the Kite shield and the Control key.
- Control key: Found on the 4th floor of site of civilization. Something is written on the back: go west from the front door.
That locked door is actually to the north-northwest.
Office key
Use the Control Key to find the Office Key. Puzzle: "The code to the door has 3 even numbers and 1 odd number. Added together they make 13. The first number is 4". Three even numbers: 2, 4, 6. 2+4+6=12. 13-12=1, that is an odd number. Therefore, the four numbers are: 1, 2, 4, 6. If 4 is the first one, there are six combinations left:
- 4126
- 4162
- 4216
- 4261
- 4612
- 4621
Actually, the top one is the right one: 4126.
- Office key: Found on the 4th floor of site of civilization. Tip of the key shines to the south.
Preparation key
Go to the central south door. Use the office key and the laboratory key.
Fight the boss by using the boomerang when its eye is open. Just watch out for his sucking power. Use the cape and jump/run. After a bit, you'll win.
In the back room is the preparation key.
- Preparation key: Found on the 4th floor of site of civilization. There is a room you haven't entered yet.
Find: a gold bullion, a source of power, an iron sword, an ancient coin.