> I can see how annoying you must find it to interfere with what you would like your map to look like.

No, that is not the issue at all.  The issue is that you are removing information that other people have added to OpenStreetMap.  The keys and values processed by that map style are based entirely on the keys and values present in OSM; they are a reflection of the richness of data present in OSM.  If someone removes data from OSM (as you did here) any map consuming OSM data (including mine, but also including many others) will be poorer for it.

> So how about adding fortification_type=castle? 

That would get us back to where we were (and was a combination in use for this sort of feature previously).  However, given what we now know about the source and relation between these objects here, maybe a site relation would also make sense?

Also - (1) are you going to make this change or do I need to, and (2) what about any other similar tagged objects that you removed information from in the same time?