Archaeological excavations in the last half century have uncovered much information about medieval fortifications: in particular, the earthworks which formed the defences of early castles. Broadly speaking, medieval defences were either private - protecting a castle or manor - or public as in city walls.
The earliest castles were timber structures, sitting on a mound or motte and attached to one or more courtyards or baileys. Motte and bailey were surrounded by a ditch. Most mottes were entirely man made, an impressive enterprise when one considers that they could be up to 10m high and massive enough to contain a building which could be two or even three storeys high. Generally, material for the mound came from the excavation of the surrounding ditch. There were exceptions; mottes were built over Bronze Age round barrows; others were founded around rock outcrops or other natural features.
Excavations have revealed that some mottes were constructed systematically from horizontal layers of material, including clay, gravel or chalk. The Bayeux Tapestry shows Hastings Castle on a mound made up in this fashion. Calculations have been made to try to ascertain how long it took to build this type of mound. The motte at Lodsbridge in Sussex stands between 5m and 6m high and has a base diameter of just under 40m. It is estimated that 50 men working a 10 hour day would have taken 42 days to complete it.
The other type of private defence was the ringwork - some of which was later converted into motte and bailey castles. Ringworks varied in shape but were generally round enclosures with earth banks and a ditch. They were quicker and cheaper to erect than the motte and bailey defences, and in some locations the Norman invaders built on existing Roman or Saxon fortifications. But from the evidence of existing sites it seems that in Britain the motte and bailey layout was preferred, and when timber was replaced by masonry, many mottes were enlarged and strengthened to take the extra load.
There may have been as many as 200 defended towns in Britain in the Middle Ages and the cost of construction was often met through a murage tax levied on goods brought into the town. In some cases Roman or Saxon defences were upgraded, but from the 13th century, stone walls with towers and gatehouses were built to enclose larger towns.
The use of stone was not universal, however. Tonbridge in Kent was defended with earth and timber walls, probably built around 1260, and some 60 years later the town received a murage grant, presumably to repair and improve them.
Many town walls vanished in the late 18th and 19th centuries as the urban population grew and towns expanded. In most cases, the medieval castle and its defences had been abandoned for something more comfortable several centuries previously.