Undated photograph. The twin mansions cover over 11 acres of gated property. SWNS/Zenger/The Jills Zeder Group, 1 Oak Studios The 14,484-square-foot home is situated on more than three acres.

At Glyn Cywarch, in northwestern Wales, romantic silhouettes—and a restless sense of play—collide a grand and storied setting. Yet, as Amanda Harlech writes, her family’s 400-year-old estate ...