It both protected the advances already made to the north, and helped to dominate the sea coast.

It sheltered the western marches, beyond which lay savage Wales--and danger.

It provided a means of getting to the Severn, round which lay the great Roman roads then coming into existence, and made possible the great waterway to the heart of England--through the Severn and its tributaries.

It brought the east and the west together by the swiftest and easiest ways known to those times.

And, finally, it provided means of descent on London and all the expanse of country watered by the Thames.
"With such a centre, already known and organised, we can easily see that each fresh wave of invasion--the Angles, the Saxons, the Danes, and the Normans--found it a desirable possession and so ensured its upholding.