Garamsythe Waterway
Sage Knowledge 42 of 78
“An underground waterway running deep beneath the Royal City of Rabanastre. Built and rebuilt over the centuries, no one alive knows all of its labyrinthine ways. Some are said to lead even to the royal palace.
There are stairs leading down to the waterway in Lowtown, but they are normally barred to keep unwelcome visitors down in the sewers where they belong.”
Rabanastre
Travel back to Rabanastre once you have obtained the Sunstone in the Giza Plains. Return to Old Dalan in Lowtown to move on to the next part of the story.
Remember not to open the treasure chest outside Dalan’s Place if you want to obtain the 
Zodiac Spear later on (and remember that this note about obtaining the Zodiac Spear does not apply if you are playing The Zodiac Age PlayStation 4 version of the game).
There is a short cutscene just outside of Old Dalan’s House where Penelo gives a heartfelt speech to Vaan and leaves the party. Vaan is on his own once again.
Enter the house and speak to Old Dalan. He will give Vaan the Crescent Stone and will direct him to go to Storehouse Five to gain entry into the Royal Palace.
There is a short cutscene that takes place after you exit Old Dalan’s Place involving Vossler and the Remnants of the Order before the game returns to Vaan in Lowtown.
Use the map and screenshot below to help you find Storehouse Five which can be found in the Northern Sprawl of Lowtown. Take a left (heading north) when you exit Old Dalan’s House.
Kytes will initiate a conversation with Vaan as soon as he attempts to enter the Storehouse. He will help Vaan gain entry through the locked door on the left, which is the one that leads to the area that Old Dalan spoke of, and he will also give Vaan a few items to see him off on his journey – 2 
Potions and 4 Phials of 
Eye Drops.
Enter the doorway on the left which leads down in to the Garamsythe Waterway.
Garamsythe Waterway
Central Spur Stairs
There is a Save Crystal immediately to the left of the entrance to the Garamsythe Waterway. Use it to save your game before proceeding. Head west, past the Save Crystal, and up the stairs into the next area.
The passageway leading east away from the Save Crystal leads to a dead end but there may be a treasure chest at the end of it if you feel like exploring.
The pathway through the Garamsythe Waterway is extremely linear – there are very few ways for you to get lost. In the first area, make your way northwest, crossing the bridge and into the next area. Use the map below to assist you.
North Spur Sluiceway
Once again, continue along the linear pathway, but this time heading to the west. There are a few areas on the south side of the map where treasure chests are likely to spawn.
These treasure chests have a fairly good chance of containing some good items (an 
Oak Staff and a 
Mage Masher to be specific) though they may not have randomly spawned and they may not contain those specific items depending on how lucky you are.
Don’t fret if you do not get them during your playthrough as they are not that important and you can purchase them later on.
Continue along the pathway using the map below to assist you until you reach the northwest section of the zone with the stairs leading up to the next level.
Approach the stairs and Vaan will mention that the Palace should be right at the top.
Choose to “Climb the stairs” to continue with the story and move into the next section of the game.
(Click on the map to enlarge)