Can you stack multiple Impaling Structures in the same hex?

We agreed in playtesting that, like you say, it would take multiple turns to do it, but there was no reason you couldn't do it if you were so inclined.

And the balance issues caused by it are obsolete now! There was a time you could stack all of them up on each other, let wraith jump to the top of it (they only attacked allies) and kill off everyone else on the map (she could ignore range completely.)

fun times!

I don't know. You try standing on top of a colossal ice pillar and not having balance issues.

So this raises the question, would an opponent stepping onto the multi-stacked impaling structure trigger every single attack of the hazard piled there or would it only trigger one attack?

I think so. It's worth noting that it raises the elevation of the hex itself, unlike omnitron who kind of floats above it, so they would all make range.

You can stack them in one action.

each one must be adjacent to another one so you could go:

AZ - IS - IS

Then go back towards AZ

AZ - IS2 - IS

Then go back again if you have Fueled Freeze.

AZ - IS2 - IS2

You can also end up with

AZ - IS - IS2 -IS

Or several other ways.

This works insanely well to block LoS lines for Baron Blade and Omnitron's drone launching.  When you raise a street level to height of 3 it messes with people.