Complex sentence: These passed by marriage to the Courtenay family, who rebuilt the castle as a lavish but defensible country retreat, until in 1539 Henry VIII seized the estate, and had Edward Courtenay executed for treason.
Split into simpler sentences: These passed by marriage to the Courtenay family, who rebuilt the castle as a lavish but defended country retreat. Then, in 1539, Henry VIII seized the estate and had Edward Courtenay executed for treason.