Udny Castle, which still stands occupied to this day, is the most obvious evidence of the prominence of this family. The castle, lying about four miles south of Tarves, is a massive fortalice dominating the surrounding countryside. It is believed that building was commenced in the first quarter of the fifteenth century and was completed and embellished over the next three generations. The costs of building it nearly ruined the family, with extensions being added in the late sixteenth century and gables in the early seventeenth century. During the reign of Mary, Queen of Scots, the Laird of Udny raised his followers in support of the Queen’s cause; they were equally prominent in their adherence to Charles I against the forces of the Covenant. In 1634 John Udny of Udny acquired property in the Belhelvie area and went to live at Knockhall Castle, while Udny Castle was sold, or leased, to William Seton of Mounie of the family of Meldrum, when he married the heiress Helen Udny. Later, the Castle had been left abandoned, but the Udnies returned 100 years later, renovating and later extending the old keep. The descendants of the family still retain ownership and occupancy of the old Castle that bears their name.

In 1634, John Udny of Udny acquired property in the Belhelvie area and went to live at Knockhall Castle and left the Udny Castle to William Seton who married the heiress of Udny, Helen Udny. Aberdeenshire Courts records for 1634 note an Exhibition:

John Seaton of Auquhorthies and George Seton, his eldest lawful son by his wife Helen Leyth v. Mr. Alexr. Davidson, Advocate in Abd. and Wm. Seton of Udny, for his interest. The writing of which exhibition was sought was a contract dated 30 May, 1634, whereby Wm. Seton, with consent of his wife, Margaret Grahame, bound themselves for onerous causes to convey to the Pursuer and his son in liferent and fee respectively, the "Maines of Udny with the towr fortalice maner place ;" &c. , the lands of Kilmortoun, Milne of Udny, the " heretabill richt libertie & priviledge of the mercat callit Crystes fair and the advocatione donatione and richt of patronadge of the Kirk of Udny callit Crystes Kirk and Chappellenrie of Udny," &c. The deed was exhibited but a motion made for registration was resisted and the case seems to have been subsequently advocated to the Court of Session.

The castle was attacked in 1693 and captured by Covenant troops under the Earl of Errol and the Earl Marischal. Just a year later, the castle was again attacked and was surrendered by Lady Udny but, as on the previous occasion, her husband, the Laird escaped.

Alexander, the twelfth of Udny, has become famous through his employment of James Fleming, renowned as the last professional jester to be retained as part of a nobleman's court. Despite James Flemings' position in the Laird of Udny's court, it was he who saved the family from certain death during a fire in 1734 which destroyed Knockhall Castle.