Not much I can do about the unused space really. Though the irregular shape, probably so. I wasn't sure on how to go about the shape of the area itself, and didn't just want to go with a flat square or rectangle shape. Hmm...
Ahhh elevation issues. Those are always tricky to deal with. I'll have to deal with those next time I get around to mapping. I'm assuming you're talking about the walls on the right side of the map (since the left side doesn't really have much touching the base floor that doesn't go up to the next floor)?
EDIT - I suppose I could fill in some empty spaces with tables and books on them or something. Hmm...I'll think of something on that front. Does everything else look fine to you at least?
Oh yes, and after defeating the boss, there will be a shortcut in the bottom of the map that'll open up for the player so they don't have to backtrack all the way through (and it'll stay there for the rest of the game).