What doesn't make sense?

The private estate was opened to public visitation 50 years ago, but the walled garden was not. Did y'all never have one of those rooms that you weren't allowed to play in, or friends with such rooms?

It explains it in literally the very next paragraph:

The walled garden has remained private to the estate's owners, the Lansdowne family, for centuries but it will now be open to offer a glimpse of the "rich history and ongoing legacy" of the estate.

Ewwwww Calne. The snobby suburbia of Chippenham.