Wikipedia has 38 castles designated as having type=Non-Movable. For example, Bellver Castle, Barxell Castle and Alcázar of Jerez de la Frontera. All of them are located in Spain. When you search Wikipedia, and the web, for a definition, nothing comes up.
I'm sure somebody wants to respond "'it's because the castles can't be moved"; thanks, please don't bother to post that. All castles are potentially movable (remember, they moved the London Bridge), but there must be something different about these particular castles.
I suspect it is either a mistranslation from the original Spanish, or else a very, very specialized term. Do historians have an answer?
The "non-movable" category doesn't correspond to the fact that they are castles but with the fact that they are Spanish Property of Cultural Interest.
Non-movable properties of cultural interest are things like monuments, gardens and historical sites.
This is a translation of the Spanish word "inmueble" which is usually used to refer to real state but it can also be used to mean "things with can't be moved"