Question: What is the main industry of the town that has a ruined L-plan keep in an ancient Scottish village?

Context paragraphs:
Preston Tower: Preston Tower is a ruined L-plan keep in the ancient Scottish village of Prestonpans.  It is situated within a few metres from two other historic houses, Hamilton House and Northfield House.

Prestonpans: Prestonpans is a small fishing town situated to the east of Edinburgh, Scotland, in the unitary council area of East Lothian.  The 2001 census recorded that the town had a population of 7,153, but many more people have moved to the area since many new houses have been built.  It is the site of the 1745 Battle of Prestonpans, and has a history dating back to the eleventh century.  The town boasts some impressive examples of historical architecture, such as the Preston Tower and the doocot and the local Mercat Cross, which is the only one of its kind in Scotland which remains in its original form and location.  The town is also credited for achieving the title of "Scotland's Mural Town" with many wall murals reflecting the town's colourful past.

Almond Castle: Almond Castle is a ruined L-plan castle dating from the 15th century.  It is located 3 mi west of Linlithgow, and north of the Union Canal, in Falkirk, Scotland.  It was known as Haining Castle until the 17th century.  The structure is unsound.  It is protected as a scheduled monument.

Tranent Tower: Tranent Tower, is a ruined L-plan tower house dating from the 16th century, in Tranent, East Lothian, Scotland.  The remains are protected as a scheduled monument.

Trabboch Castle: Trabboch Castle (NS 458 221) is a ruined L-plan tower house in the old Barony of Trabboch, Parish of Stair, East Ayrshire, Scotland.

Burning of the Clavie: Burning the clavie is an ancient Scottish custom still observed at Burghead, a fishing village on the Moray Firth.  The "clavie" is a collection of casks split in two, lit as a bonfire in the evening of 11 January, i.e. New Year's Eve (in Scotland, Hogmanay) by the Julian Calendar.  One of these casks is joined together again by a huge nail (Latin "clavis"; hence the term, it may also be from Scottish Gaelic "cliabh", a basket used for holding combustibles).  It is then filled with tar, lighted and carried flaming round the village and finally up to a headland upon which stands the ruins of an altar, locally called the "Doorie".  It here forms the nucleus of the bonfire, which is built up of split casks.  When the burning tar-barrel falls in pieces, the people scramble to get a lighted piece with which to kindle the New Year's fire on their cottage hearth.  The charcoal of the "clavie" is collected and put in pieces up the cottage chimneys, to keep spirits and witches from coming down.

Whitefield Castle: Whitefield Castle is a ruined L-plan tower-house on the hill above the village of Kirkmichael in Strath Ardle, Perth and Kinross, Scotland.

Auldhame Castle: Auldhame Castle is a ruined L-plan tower house standing on a ridge above Seacliff beach, about 3 miles east of North Berwick in East Lothian, and less than half a mile from Tantallon Castle.  The castle was built in the 16th century, probably by Adam Otterburn of Reidhall, Lord Provost of Edinburgh.  It consists of a three-storey main block with a projecting stair-tower.  Part of a vaulted basement remains, but the upper floors are mostly gone.

Cessford Castle: Cessford Castle is a ruined L-plan castle near the village of Cessford, equidistant between the Royal Burgh of Jedburgh, and the Burghs of Kelso and Kirk Yetholm, in the historic county of Roxburghshire, now a division of the Scottish Borders.  The Castle is "caput" of the Barony of Cessford, and the principal stronghold of the Kers/Kerrs, notorious Border Reivers, many of whom served as Wardens of the Middle March.

Cowthally Castle: Cowthally Castle, also known as Couthalley, is a ruined L-plan castle near Carnwath, Lanarkshire, Scotland.  The remains are protected as a scheduled monument.

Supporting paragraphs: Prestonpans Preston Tower
Answer: fishing