You are guaranteed to be able to reach every room in every palace. A connection between two rooms will be the same in both directions - if you enter a room and turn around you will always go back to the previous room.
Keys
It is possible that you will not be able to acquire enough keys to unlock every door. This means you may (and probably will) need to rely on the fairy spell to get past locked doors.
Fallthrough maps
Certain rooms in palaces allow the player to fall through the floor. These fallthroughs only allow the player to move through them in one direction, so they deserve special consideration. It is guaranteed that a fallthrough will always allow a player to exit the level - either through the entrance or through the exit after the boss. It is possible to get stuck in palace 4 because of these fallthroughs. If you go down a fallthrough in palace 4, it is possible that the only way to get out of the palace is through Carock. If you do not have the reflect spell, then you will be unable to defeat Carock and will have to Up+A to leave the palace.