Ahh.... I do love being able to practice my necromantic arts without persocution XD
Anyway, as I see it, the letter Leene wrote was intended to mean that she would keep everyone unaware of it's presence, as it was meant to be guarded until the time heroes returned to claim it; that as reward for finding it, they were the rightful recipients of its glory...
Another thing is, who's to say that there aren't more than one treasury? In all actuality the scale of that castle is
extremely tiny compared to a real one, its more along the lines of a moderate sized mansion... Though, in context of the game it was meant to be a massive wondrous palace full of rooms and such. For instance, a typical castle has enough room to board all of the staff that are required for its upkeep, and if that castle is a defensive position (usually they are) room for the knights as well. As we all can clearly see, it isn't nearly big enough for that.... oddly enough, there are far more storage rooms, filled with seemingly useless junk, than there are beds. We see this as we escape from the prison wing, speaking of which, where are all of these "wings" located? Surely they are somewhere, though note that they only appear after they become relevant to the story. If I remember correctly, the treasury that contains the rainbow shell doesn't even exist until after it is found. While the same is theoretically true of the judicial wing, we at least get a word that it will come to exist. No one ever mentions building a new treasury in 600AD do they?
I didn't say they were being paid by Guardia in exchange for protecting the treasures...I just said they were allowed to populate the treasury...Not that they control them, it's just that the treasury was sort of just left in ruin...to keep in more in secret...?
As for what you're talking about Boo...Yeah, I don't think time travel works that way in the series...>_>
Ah, that makes more sense. That was no shiny treasury...When I think treasury, I see

Note the pirate dude there... Pirates didn't really think about making their treasure seem worthless in order to protect it, they simply hid it or kept it with them.
If something was REALLY important, wouldn't the best protection be to make it seem unimportant... like say, the object in the treasury at Guardia? OR perhaps Leene accomplished that guise in order to make the inhabitants of the castle dismiss it as "just another storage room filled with crap". That would explain why it isn't upkept... its sort of like attic space or basement junk rooms.... no one goes in there because they know its all just junk...
But, ever notice you find interesting things when cleaning out junk rooms after a while...? Sometimes, things you didn't know you had...? and often there may even be animals or bugs living there that aren't in the rest of the place. (this coincides with the monster infestation, though they could have been sent there to raid the room during the trial... the trial was in fact, a diversion... this is what I believe.)