Question: Walter Scott of Harden (before 1563 – 1629), also known as Auld Wat, was a notorious Border Reiver, what's the name of the 16th century tower house, about 3.5 mi west of Hawick, Scottish Borders?

Context paragraphs:
Dumbreck Castle: Dumbreck Castle was a tower house, dating from the 16th century, around 3.5 mi west of Ellon, in Aberdeenshire, Scotland.

Walter Scott of Branxholme and Buccleuch: Sir Walter Scott, 1st of Branxholme, 3rd of Buccleuch (c. 1495 – killed 4 October 1552), known as "Wicked Wat", was a nobleman of the Scottish Borders and the chief of Clan Scott who briefly served as Warden of the Middle March.  He was an "inveterate English hater" active in the wars known as The Rough Wooing and a noted Border reiver.  He was killed on Edinburgh High Street in a feud with Clan Kerr in 1552.  His great-grandson was Sir Walter Scott, 1st Lord Scott of Buccleuch, the "Bold Buccleuch" (1565–1611), a border reiver famed for his role in the rescue of Kinmont Willie Armstrong.

Waverley Route: The Waverley Route was a railway line that ran south from Edinburgh, through Midlothian and the Scottish Borders, to Carlisle.  The line was built by the North British Railway; the stretch from Edinburgh to Hawick opened in 1849 and the remainder to Carlisle opened in 1862.  The line was named after a series of novels by Sir Walter Scott.

Barholm Castle: Barholm Castle is a tower house located five miles south-west of Gatehouse of Fleet, in Dumfries and Galloway, southwest Scotland.  The tower dates back to the late 15th century, and it was a stronghold of a branch of the McCulloch family.  The present form of the castle dates from rebuilding in the 16th or 17th century, and in the early 2000s it was restored from a roofless state to residential use.  Barholm The tower is sometimes identified with the fictional Ellangowan, in Sir Walter Scott's "Guy Mannering".

Walter Scott, 1st Lord Scott of Buccleuch: Walter Scott, 5th of Buccleuch, 1st Lord Scott of Buccleuch (1565 – 15 December 1611) was a Scottish nobleman and famous border reiver, known as the "Bold Buccleuch" and leader of Kinmont Willie’s Raid.  Scott was the son of Sir Walter Scott, 4th of Buccleuch (himself grandson of Walter Scott of Branxholme and Buccleuch) and Margaret Douglas.

Nether Horsburgh Castle: Nether Horsburgh Castle is a ruined tower house near Cardrona, in the Scottish Borders, in the former county of Peebleshire.  It is situated at the back of a farmstead, 3.5 mi southwest of the market town of Peebles, at grid reference grid reference [ NT304396] , on the A72 road.  Access may be gained by permission from the adjoining farmstead, Nether Horsburgh Farm.

Fatlips Castle: Fatlips Castle is a pele tower in Roxburghshire, in the Scottish Borders.  Situated at the top of Minto Crags, above the River Teviot, it was built in the 16th century by the Turnbulls of Barnhills, notorious Border reivers, and owners of nearby Barnhills Tower and farm.  In 1545, during the War of the Rough Wooing, the Earl of Hertford burned "Mantoncrake", or Mynto Crag.

Harden Castle: Harden Castle is a 16th century tower house, about 3.5 mi west of Hawick, Scottish Borders.  It is alternatively known as Harden House or Harden Tower.

Walter Scott of Harden: Walter Scott of Harden (before 1563 – 1629), also known as Auld Wat, was a notorious Border Reiver.

Barnhills Tower: Barnhills Tower was a 16th century tower house about 4.5 mi west of Jedburgh in the Scottish Borders, Scotland, north of the River Teviot, on the high left bank of the Craigend Burn.

Supporting paragraphs: Walter Scott of Harden Harden Castle
Answer: Harden Castle