Seems excessive, yea, but it may make cavalry/fast units much more important for attacks on these sieges (to move through the kill zone and cap the point).
There may be something with "move reinforcements" that is allowing such buildings, right off the start. The Chaos Undivided LL has a ability to buff that, but haven't looked up what it actually does. I have only done one settlement battle defense because I mostly auto resolving everything.
I haven't got too far into the game yet, as my second Chaos attempt was overwhelmed by Kislev marching on my capitol while empire and elves were going after my minor settlements (around turn 45).