Butlins Skegness has some great castles near by.
Castle Rising, Kings Lynn, Norfolk - a well preserved 12th century castle and earthwork defences built in around 1138. A royal residence and a royal mint. Between 1330 – 1358 for the former queen Isabella of France, widow of the murdered Edward II, who died here. One of the most famous 12th century castles in England, the well preserved stone keep is amongst the finest surviving examples of its kind and is surrounded by 12 acres of earthwork defences. Restricted opening times and entrance charges apply
Somerton Castle, Nr Boothby Graffoe, Lincolnshire - an earthworks and limited remains of 13th century castle. King John II of France was imprisoned in the castle between 1359 and 1360 after being taken prisoner at the Battle of Poitiers. Some prominent earthworks still enclose the site, including parts of the moat and parts of the castle walls have been incorporated into the present farmhouse.
Lincoln Castle, Lincoln, Lincolnshire - is one of the better preserved castles in England built for William the Conqueror. Stone walls were added early in the 12th century. The outer bailey stretched around the entire medieval city of Lincoln. For 900 years it operated as a court and prison, it's early prisoners suffering execution on the castle ramparts. Still home to the Crown Courts, the castle is open to the public as a museum and displays an original copy of the Magna Carta. Restricted opening times and entrance charges apply
Newark Castle, Newark-on-Trent, Nottinghamshire - is the remains of medieval royal castle. The castle belonged to King John, and it was here that he died in 1216 following his infamous "surfeit of peaches". Following the English Civil War the castle was slighted and left derelict. Some restoration of the buildings began in the 1840s
Bolingbroke Castle, Bolingbroke, Lincolnshire - the remains of a 13th century hexagonal castle built around 1220. Birthplace of the future King Henry IV in 1366. During the English Civil War, the castle was "slighted", with the towers and walls being torn down and dumped into the moat to prevent further military use. Free open access at any reasonable time.