 Castles were introduced to England by the Normans in the 11th century and their construction , in the wake of the conquest of 1066 , helped the Normans secure their new territory . Rochester was an important city , built on the site of a Roman town at the junction of the River Medway and Watling Street , a Roman road . It has long been assumed that the first castle was located next to the river , just outside the south @-@ west corner of the town walls . The conjectural site of the early castle later became known as " Boley Hill " . Archaeologist Tom McNeill has suggested that these earliest castles in England may have been purely military in character , built to contain a large number of troops in hostile territory . 
