The spot, having the waters of the rivers for a defense upon two sides of it, was easily fortified.

A castle had been built there, and, as usual in such cases, a town had sprung up about the walls.
The Danes advanced to this stronghold and took possession of it, and they made it for some time their head-quarters.

It was at once the center from which they carried on their enterprises in all directions about the island, and the refuge to which they could always retreat when defeated and pursued.

In the possession of such a fastness, they, of course, became more formidable than ever.

King Ethelred determined to dislodge them.