Sir William Borthwick of that ilk. Ambassador to England 1398-1415, granted a charter of the lands of Borthwick, Selkirkshire by the Regent Albany 1410, hostage for safe return from England of James I of Scotland 1421, commissioner to treat for the King's release 1423. married a daughter of Sir Thomas Hay of Lochorwart. [Burke's Peerage]
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BORTHWICK CASTLE
Just over a mile and a half south-east of Gorebridge, Midlothian.
Borthwick castle is one of the most impressive tower houses in Scotland and at a height of 110 feet it is the tallest.
Complete and largely unaltered during its 580 years, it was built by Sir James Borthwick (later Lord Borthwick) in 1420, on the site of an earlier motte castle known as Lochorwart, granted to him by James I. It is U-shaped in plan and has walls up to 14 feet thick at lower levels. (Note James, 1st Lord Borthwick was son of William & Miss Hay)
[Note the first Lord Borthwick was William-not James, and another site states it was built by William in 1430]