Someone mentioned this in some sort of post about Phantasy Star. I do not remember when or who. They claimed that there's a false wall (as in Maharu Cave and Tajima Cave) in the Casba Cave, and that a chest with 5000 meseta is in there.
I could go tile by tile and try to go through the wall in every direction, but that'd be extremely tedious and I'd like to avoid it. Does someone - lorenzo, maybe - have access to the treasure data in the game?