Just from reading the wider context of the passages surrounding the quote I could not rule out the writer using poetic licence, likening the rocks of the landscape to battlements.
Seeking to test this theory I looked at aerial images if Aykel, which were inconclusive.
I then did an online search for images of Aykel and found this image on a travel blog ...
"cast a bank" means to build a ramp.
This is the archaic meaning of "cast" as in item 1, subitem 9 here:
(archaic) To throw up, as a mound, or rampart.
Bible, Luke xix.48
Thine enemies shall cast a trench [bank] about thee.
And the meaning of "bank" as in a bank of something, as in item 2 here:
A long, high mass or mound of a ...
"Ruled by an iron rod" is a well-known English idiom. From the Oxford English Dictionary:
Control or govern very strictly or harshly.
‘she ruled their lives with a rod of iron’
The peasants in your quote have lives which are controlled very strictly or harshly, hence they're ruled by an iron rod. What controls their lives so much? The "soil and season" ...