Oh, okay, it's like a car game
you move unit by "jump" (with the up arrow maybe)
and you want it to crash if it is going through a tree or something else
what are you using to make ground unwalkable? doodads (trees)? terrains (water)?
EDIT:
I've made test with a trigger that simply display the value of the condition
ground > walkable
tree > walkable
water > unwalkable
limits > unwalkable
so if you use doodads, it doesn't work
it only care about tiles type