Yeah, Wrought Iron is what I meant with no proper Steel, but for that they ofcourse need to break it enough to put the Wedges in, dont they? 
And an Iron-Strength Material will be available in the late earlygame, so that is quite easy to use to break Enemy Walls slowly. Not like they care about the Structural Integrity of the Wall if they wanna destroy the Wall. If anything they wanna weaken the Integrity.
Also I just noticed you edited your previous Post. Not a good Idea when someone already left a Like on it, thinking it has already been read due to that.
The Star Forts are insane, but they are indeed kinda spammed to make sure a large Area can be kept under control, so they did not choose the wisest places for those indeed.