In 1384 Archibald attacked and took Lochmaben Castle, the last English stronghold in south-west Scotland, and in 1388 he inherited the title of 3rd Earl of Douglas. He thus became the most powerful magnate in the country, controlling lands across southern Scotland and castles including Douglas Castle and Bothwell Castle in Lanarkshire. He funded a rebuilding of Sweetheart Abbey near Dumfries, and contracted marriages for his children with the offspring of King Robert III. Although he probably spent relatively little time at Threave, he died there in 1400, the first historical reference to the castle. He was succeeded by his son Archibald, who in 1390 had married Princess Margaret, daughter of Robert III. In 1424 he led a Scottish force to fight the English in France where he was killed at the Battle of Verneuil. His wife Margaret took on the Lordship of Galloway on Archibald's death, and ruled from Threave as Countess of Douglas and Duchess of Touraine for the next 23 years, outliving her son and grandsons. In 1449 she was forced to cede Galloway to the 8th Earl of Douglas, and died at Threave soon after. Her monument stands in Lincluden Collegiate Church. A lead seal matrix made for Margaret was recovered during archaeological excavations at Threave. The seal shows her coat of arms, combining the arms of her husband with the Royal Arms of her father Robert III. Although a rare survival, the workmanship is poor suggesting it may have been a trial piece or even a forgery.