I found 1 of secret rooms at the castle by accident. One of it was at rooms below the rooms look like the church where there are two altars. if u go downstairs, go to bottom left. u will go downstairs.
the second one was at the library, where the largest book shelves, if u walk through it, u will enter another rooms.
p.s. it said there are 4 secret rooms in the castle if u read the grave stone at the first room i found.
p.s. do not enter carelessly, it an ambush.
p.s. yes, there are chest and loots