 Edmund, the Earl of Lancaster and the "capitaneus" of the royal forces in Wales, was given the Three Castles in 1267 and for many centuries they were held by the earldom, and later duchy, of Lancaster.King Edward I's conquest of Wales in 1282 removed much of Grosmont's military utility, but, under either Henry of Lancaster or his son Henry of Grosmont, the interior of the castle was modernised in the first half of the 14th century to create a suite of high quality apartments.A deer park was maintained around the castle.