Ardenian is correct. Invisible platform. Can anyone show a screeny bout the invisible platform?
Well, I don't think I was 'correct'. I simply understand the question in a way than you wanted to know an answer.
The invisible platform is located under the bridge category.
Basically, you place them like a doodad.
Then, you use CTRL + PgUp/PgDn, as Rheiko said, to change the height of the invisible platform.
Screen 1: We have a doodad, but the units simple walk 'under' it, on the ground.
Screen 2: We place some invisible platforms, but they are on the ground, so nothing changed.
Screen 3: Using CTRL + PgUp/PgDn, we move them to make units walk appropriate 'over' the bridge.
Requires some testing, but is worth the effort. You might want to place the platforms first and then move the bridge there.
Note: You have to place pathing blockers at the sides, where no invisible platform is to prevent units walking in the abyss