FFXII - Walkthrough - Garamsythe Waterway
Immediate directions on where to go; feel free to take some detours if you want to wipe out some enemies or pick up some treasures.
Turn left; save your game. Go up the stairs to new area.
Northern Sluiceway: there are 3 square-ish areas. You'll want to go to the northeast part of the eastern-most square. So in other words; go north of the first square, east through first two squares, east through 3rd square and then up. West to next area.
North Spur Sluiceway: Go west. West West West! Up the stairs, then down the stairs. When it seems you can't go west anymore, go north; up a couple of flights of stairs, and you're at the palace cellar!
NOTE: no turning back once you climb the stairs, so if you want to go back to the city and buy or do anything, it'll be your last chance for a few hours.