I am sorry, I guess I am just the only one who didn't get it. I have been scapping every walls of those damned ruins without success but I can't find the hidden passage. Is it on the lowest floor (B2) ? on the B1 floor ? F1 or F2 floor ? Could you tell me the name of the stairs in question on the floor in question (example : Floor F1, stairs B2_B) ? My dumbass brain couldn't find it. I have tried every stairs and every wall next to every stairs, east and west stairs, walls, on this godforsaken map for hours on end, my eyes searching for any pixel of blue floor without success (and yes, map stairs and doors are blue, I know). I feel like I am going crazy x'D. East stairs B2 underground, west wall touching the blue floor surrounding the stairs leading down should have been easy, but I just can't find anything. At this point, it's rage,hatred at this level and stubborness alone that keeps me searching ;/
EDIT: for those who could help me, I am trying to reach this girl here in the Susano underground