The castle ruins provide not only a viewpoint of the Conservation Area, but a suitable place from which the visitor can start to explore the ancient borough. As the winding path up the castle mound is followed, the whole of Plympton St. Maurice is revealed. The pattern of the streets around the Church takes shape with Plympton House beyond. There are views of old rooftops, back gardens, and even a farmyard below. To the north the land slopes up to the Ridgeway, and, to the south, beyond the houses that pack tightly between the Castle Green and the Longbrook stream, fields and woods reach up to the skyline. 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. Timber reinforcing within the walls has since decayed, leaving "tunnels" in the stonework. As can be imagined, these "tunnels" have given rise to fanciful tales of secret passages! 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. This path is lined with 31 lime trees planted in 1898. In 2003 a parallel line of lime trees was planted to celebrate the Queen’s Golden Jubilee. It is used'as a short cut to the Ridgeway and makes a very pleasant rural walk, especially in July when the lime trees are in flower. The field nearest the castle is called Castle Hayes. Here cattle belonging to the castle were grazed and cows are still sometimes pastured here. In this field the covered remains of St. Thomas or Pound Well can be seen. Would the inhabitants of the castle have relied on this Well for their water supply?