The Normans had begun to make incursions into South Wales from the late 1060s onward, pushing westwards from their bases in recently occupied England.

Their advance has been marked by the construction of castles, frequently on old Roman sites, and the creation of regional lordships.

The reuse of Roman sites produces considerable savings in the manpower required to construct large earth fortifications.

Cardiff Castle is built during this period.

There are two possible dates for the construction; it is possible that William the Conqueror built a castle at Cardiff as early as 1081 on his return from his pilgrimage to St. Davids.

Alternatively, the first Norman fortification may have been constructed around 1091 by Robert Fitzhamon, the lord of Gloucester.

Fitzhamon had invaded the region in 1090, and will use the castle as a base for the occupation of the rest of southern Glamorgan over the next few years.

The site is close to the sea and can be easily supplied by ship, is well protected by the Rivers Taff and Rhymney and also controls the old Roman road running along the coast.

Cardiff Castle is a motte-and-bailey design.

The old Roman walls had collapsed and the Normans used their remains as the basis for the outer castle perimeter, digging a defensive trench and throwing up a 27-foot (8.2 m) high bank of earth over the Roman fortifications.

The Normans further divided the castle with an internal wall to form an inner and an outer bailey.

In the northwest corner of the castle a wooden keep is constructed on top of a forty-foot (twelve meter) tall earth motte, surrounded by a thirty-foot (nine point one) wide moat.

The motte is the largest built in Wales.

The overall size of the castle is around eight and a quarter  acres (three point thirty-four hectares), with the inner bailey being around two acres (point eight-one hectares) in size.

Mills are essential to local communities during this period, and the castle mill is located outside the west side of the castle, fed by the River Taff; under local feudal law, the residents of Cardiff are required to use this mill to grind their own grain.

The conquered lands in Glamorgan are given out in packages called knights' fees, and many of these knights hold their lands on condition that they provide forces to protect Cardiff Castle.

Under this approach, called a castle-guard system, some knights are required to maintain buildings called "houses" within the castle itself, in the outer bailey.

Anglo-Saxon peasants settle the region around Cardiff, bringing with them English customs, although Welsh lords will continue to rule the more remote districts almost independently until the fourteenth century.

Cardiff Castle is a Marcher Lord territory, enjoying special privileges and independence from the English Crown.

The medieval town of Cardiff spreads out from the south side of the castle.