Basically, I completed all areas except for the lab. I found all skills (including that of the Blue Spirit) and found 5 hidden rooms.
The "specific wall" I mentioned was that one room before the Blue Spirit place at the beginning of the game.
This is strange, I thought that to enter the last part, I had to enter all the hidden rooms in a single attempt without dying or using teleport or saves from rabbits. Well, if you saying that I dont need to do this, I'm happy again :)