I didn't think the Tower of Fate was too bad, what with the replenishing health
and yeah, the Lich yard is probably my least favorite stage
I don't pretend to be a master or anything, but the most frustrating boss for me was without doubt Plague Knight
his jumpy shenanigans were for some reason much more treacherous than Black Knight's or King Knight's.
In The Halls of The Usurper, King Knight's Stage's theme is my favorite music from the whole score
which is incredibly convenient, since it's where I go when I need to farm gold.