Well with my research I realized that at the beginning of the period that I want to play (say between 1515 and 1575), there was still very little castles with walls and towers built on stones basement !
In addition in the Mori region - Chugoku - population density and standard of living were lower than in the plains of central Japan, because Chugoku was (and still is!) a very mountainous region!
In fact there were mostly Yamashiro in wood and plaster, consisting in several plateaux developed (and fortified) connected to each other!
It's very interesting !
In fact I am going to build a whole series of towers, walls and simple buildings, in foamboard and balsa, flexible: it is relatively long to do, but not very expensive, so little by little there is way to do something very nice!
A prototype: a wall portion 20 cm long:



Of course my stone-basement castle remains quite valid for the most powerful fortresses or the later period, say after 1550 ...