Early settlers were attracted to this sheltered and fertile valley, and the low spur projecting westward towards what was once a tidal creek proved an ideal site for a Celtic fortification. The Saxon name Plymentum for the village which grew up here probably derives from the Celtic Pen-lyn-don (fort at the head of a creek). It is thought that the Saxons constructed a wooden fort here as protection against enemies such as the Danes who landed on the south Devon coast in 851.
A son of Alfred the Great acquired the manor of Plymentum in 904. It was known as Plintona at the time of the Doomsday Survey in 1086, but it became Plympton Erle (Earl) after the manor had been granted to Richard de Redvers, who was made Earl of Devon in the reign of Henry I.
The castle we see now was probably built by Richard's son Baldwin. It was a motte and bailey castle. A fourteenth century drawing shows a roofed round stone tower, some thirty feet high, on the motte, or mound. The living quarters and storehouses were in the bailey, but these buildings have vanished. The course of the surrounding moat can still be traced: in Henry VIII's time it was stocked with carp. There was an entrance at the west end of the bailey, where a drawbridge crossed the moat from a fortified gateway or barbican. The house called Castle Barbican now stands here. There was probably another entrance on the north side leading to the path across the Pathfields.
The castle is remarkable as having undergone two sieges. In 1136 Baldwin de Redvers, lord of the manor, rebelled against King Stephen who sent two hundred knights and archers to take Plympton.
In 1224 Fawkes de Breaute, who had married a Redvers widow rebelled against Henry 111. Henry sent the Sheriff of Devon to attack the castle with ten knights, sixty serjeants and three siege-engines. The castle surrendered after a fifteen day siege.
In 1370, the Black Prince, returning from the wars in France, paid a visit to the partly restored castle.
It was used as a stronghold for Royalists in the Civil War, when they besieged Plymouth from 1642 to 1646. Prince Maurice had his headquarters here with five regiments of horse and nine of foot soldiers, but it was taken by the Parliamentary force in 1644. Cannon and musket balls, dating from this period, have been found in local gardens.
The bailey, or castle green, continued to play an important part in the life of St. Maurice. In the eighteenth and nineteenth centuries, circuses and fairs were held here regularly. There was maypole dancing, wrestling, cock-fighting and prizefighting. There were also "grinning" matches which were probably similar to the "gurning" competitions still held today in the Lake District where villagers compete to pull the ugliest face. By tradition the newly elected mayor provided a bull for baiting, and at one time there was an annual "jingling" match between "smart dashing lads" for a hat and a guinea prize. Jingling was a popular game at village fairs in the eighteenth century. All players were blindfolded, except one who ran about ringing a bell in each hand, while the others tried to catch him. However, some of these diversions got out of hand and, after fighting broke out at the annual fair on May 29th, St. Maurice's Day, 1786, the fair was stopped for twenty years.
Early in the nineteenth century Lord Boringdon, who then owned the castle, sent a plough and two horses to plough up the green. The mayor and members of the corporation removed the horses and burned the harnesses and plough, and the villagers danced in celebration. In 1896, Lord Boringdon's successor, now Lord Morley, gave up his rights to the green to the parish council.
After the victory at Waterloo in June 1815, there was "feasting and frolicking" and in 1856 at the end of the Crimean War, all the inhabitants of Plympton sat down to a meal laid out on tables on the green. Royal weddings and jubilees were celebrated there, and each Empire Day, in the early years of this century, school children went "ring singing" on the mound, or ring, as it is locally called.
Since the establishment of the Plympton St. Maurice Civic Association in 1968, a fair, The Lamb Feast, has been held on the castle green in June.