Vaan will enter the Royal Palace from the Garamsythe Waterway winding up in the Cellar Stores. There is a brief intro video prior to the next scene which stars Balthier and Fran (the two new characters who will be joining your party shortly).
Save your game using the Save Crystal and then examine the bright green and gold Urn to obtain the map of the Royal Palace of Rabanastre.
The game will warn you that it is not easy to leave this area but that warning will have very few implications (especially armed with this walkthrough). Go through the doors and into the actual Cellar.
Open all of the treasure chests in the Cellar with the exception of the two mentioned in the warning note above.
There is an extremely rare spear that you can obtain under very specific conditions called the 
Zodiac Spear. One of the conditions is that there are four treasure chests found throughout the game that you cannot open under any circumstance. The second of these treasure chests is the one located in the southeast corner of the Cellar of the Royal Palace. Do not open it – the items that it contains are inconsequential. One contains an 
Elixir which is a nice item but it is not worth ruining this quest over. If you obtained the Elixir you should restart the game from your most recent save file.
The Zodiac Age: This warning can be ignored in the Zodiac Age version of the game as the Zodiac Spear is no longer available in the Necrohol of Nabudis. You are free to open up all of the treasure chests in this version of the game.
Approach the crowd gathered on the north side of the cellars for a short cutscene involving the Imperial Guard.
Approach the Imperial Guard near the stairs first and then speak to the Palace Servant (the red Bangaa wearing yellow clothing) standing near the guard twice. The Palace Servant will tell you that you can get past the guards by “calling them over”.
The Imperial Guards are ridiculously easy to maneuver past – press the 
Button and then run around the guard, up the stairs and into the Lower Halls.
The only way to get through the Lower Halls of the Palace is to use the “calling the guards” over strategy to move the guards out of the way. This will allow Vaan to travel down certain paths and hallways unimpeded.
Take the pathway leading to the right (south) around to the east side of the zone and call the guards standing at the intersecting hallways down towards the south.
Run away quickly and back to the intersection near the entrance (to the west of the Hawk Signet) and call the guards that are standing to the north down towards you (leading them south). Run eastward past the Hawk Signet and around to where the first set of guards were standing.
Travel north from there and then head west back into the middle of the room. You should pass over a Lion Signet on the floor.
Choose to use the Crescent Stone while standing on the Lion Signet to illuminate the sun’s light at the end of one of the pathways leading to the secret passage.
Travel west again and call the guard to the north down towards Vaan. Head back east over top of the Lion Signet and circle around to the northwest corner of the room.
The sun’s light is at the end of the hallway to the west as shown in the screenshot below. Go through the doorway and into the Secret Passage.
Vaan will immediately be locked into the Secret Passageway as soon as the team enters.
Travel to the far end of the room and search the left side (south side, although there is no map) for a hidden switch as shown in the screenshot below. Go through the newly unlocked doorway on the other side of the room and choose the option to examine the suspicious-looking door.
A short cutscene will take place involving Balthier and Fran and their first meeting with Vaan.
Climb the stairway to the top of the room for another video involving all three characters as they exit the Royal Palace and head back into the Garamsythe Waterway.
Use the map below to help navigate:
(Click on the map to enlarge)
