Wood is easy to acquire - just cut down trees - which the troops could do on the scene
if needed. Wood also has some "give" which would allow the recoil of the gun to be
absorbed by the structure.
What is function of the boards at an odd angle in the front of the picture?
They seem to be leading off the structure, perhaps to some other part of the gun
emplacement. To be sure, look at
other views of the "Dictator."