There were at least two books, although I only recall the plot of the first one. They were paperbacks, probably about octavo dimensions, fairly short as I remember them, under a hundred pages. I read them in the early 1990s in Kentucky. They were part of the family library, but unfortunately no longer seem to be there.
The books featured a group of children, likely siblings, mixed sexes and Caucasian, living in a castle that was being renovated in contemporary times, I think by the children's parents. I remember there were workers onsite. The children learn that there was likely a secret room in the castle (I think it was a combination of stories and them working out that the dimensions of the castle suggest a hidden space) with treasure in it, and some of the workers plan to find and steal it. The children learn more about the plans of the workers by dangling a microphone off of a fishing pole (I remember there was an illustration of it) attached to a tape recorder, and recording from a floor above. When they do find the secret room, I remember that it involved a door that opened onto a set of stairs going down.
What made a real impression on me at the time was that the author provided a lot of little details about castle construction, particularly how it applied to defense. Chiefly, I remember them mentioning flat roofs for archers to fire from and that spiral staircases always ascended clockwise so that defenders had their right sword hand free while attackers would have to use their left. There were illustrations, but my memory of them are of something like technical diagrams, floor plans of the castle and the aforementioned illustration of their eavesdropping device.
I have paged through Five Have a Wonderful Time (suggested in the comments since it involves a group of children, a castle, and a secret room), but it is not a match.