As I'm sure everyone on this list knows, castles are always relevant. So one thing that occurs to me is that in some instances buildings will have been converted into prisons (eg: Lancaster Castle). It might be worth creating separate Wikidata items where that is the case and linking the two, as otherwise you may have an inception date in the 1090s for a prison. That's the approach being used for data from the Atlas of Hillforts, as where a castles was built on a hillfort (eg: Caus Castle) you would end up with two inception dates which is a bit messy.

Richard