For gosh sake, IT'S A CASTLE !!!, or it is something pretty much comparable. So we keep it.
Many ancient castles may meet WP:GEOLAND as legally (or illegally) populated places. Castles were often at some distance from the nearest non-castle settlement and housed hundreds of people (in the castle, and around the castle in ramshackle huts and buildings). Castles, or castle remains, will typically have coverage in history, geography, and touristy books.
Or it's a public museum, say. Museums are cultural institutions, and they publish about their holdings and sometimes more, and they are public attractions, and the touristy books and many other sources cover them.
Or it is a tourist attraction or some other kind of public attraction. A theme park, or a beach, or a monument, or an art gallery, or a botanical garden, or an aquarium, or a zoo. Apart from the opening hours, which seem to get reported widely (not saying Wikipedia should report on the opening hours), are descriptions of their features and why people should visit them. They are covered in local and regional and national and international and galactic guidebooks and many other sources.
The point being, for any sufficiently large and touristy place, it really isn't that hard to find sources to write an encyclopedia article about it. Some AfD participants have complained about the quality level of this essay, when it has been linked, including one going as far as calling this "a piss-pathetic essay". In an AfD which concluded "Keep", of course. Because the place was a castle or a museum or a tourist attraction or another public attraction which obviously should be Kept. And it is not necessary to explain a lot more.