From the FAQ:
Q: What, exactly, is an empty space?
A: An empty space is a space devoid of all obstacles and
all figures. It may contain a potion, treasure chest, familiar,
or other non-obstacle token. The sole exception is that
corrupted terrain does not, in its own right, make a space
It sounds like an OL could play Crushing Block directly onto a glyph. I assume stairs count as an obstacle, or that could be even more damaging to the quest.
Is a glyph an obstacle?