HISTORY

The Romans built a small fort here in the 1st century CE, where the Gloucester - Carmarthen road crossed the Taff, altering it several times before creating the present stone-enclosed site.
The town began its continuous existence with the arrival of the Normans in the 11th century. The Norman landowner Robert Fitz-Hamon built a fortification within the remains of the Roman fort, possibly as early as 1081. 
Cardiff Castle became the base of the lords of Glamorgan, governing the county on behalf of the English crown for the next 450 years. By 1150 a stone shell keep (a masonry replacement for a wooden palisade crowning the motte) was erected on the mound—one of the finest surviving examples of its type in the country.

(Courtesy of the Encyclopaedia Britannica)