Twin Silo Farm, a 100-acre estate in Doylestown, Pa., dating back to 1710, has one of the few remaining stone homes built when Pennsylvania was still a colony defined by the William Penn land grant. The house, which started as a single-room farmhouse in the 1700s, has been expanded through the centuries to its current state—a 15,000-square-foot oasis situated near 50 splendorous acres of gardens. The main home features restored original hardwood floors and deep sill windows, exposed beams, a slate roof, custom…