Hatfield is a town and civil parish in Hertfordshire, England, in the borough of Welwyn Hatfield. It had a population of 29,616 in 2001, and 39,201 at the 2011 Census. The settlement is of Saxon origin. Hatfield House, home of the Marquess of Salisbury, forms the nucleus of the old town. From the 1930s when de Havilland opened a factory until the 1990s when British Aerospace closed it, aircraft design and manufacture employed more people there than any other industry. Hatfield was one of the post-war New Towns built around London and has much modernist architecture from the period. The University of Hertfordshire is based there.