Spell "Spider's Web" in the "Book of Illusions" doesn't work over grassy areas

[aeoo]

This one might be "as designed" but it isn't intuitive to me. I don't understand why would a spell like "Spider's Web" not function in a natural area.

Further, I noticed that the normal open floor tiles tell me that traps may be set on them, but grassy areas (including "open trail through grass" and possibly other types of areas) don't say that traps can be set on them. However, I can arm traps on grass without any problems. But when I subsequently disarm those traps, the tile under the trap is shown as "open floor" instead of grass.

I think areas like "stone bridge" and "open trail through grass" and all the other non-extreme tiles should accept both traps (and say so in their descriptions) and effects like the "Spider's Web." I can understand lava, water, acid, solid rock and similarly extreme tiles not being amenable to traps or spider's webs.