11. READING.--There is no "work" mentioned by the _Anglo-Saxon Chronicle_ at this place, which the Danes made their headquarters in 871, but we add it to the list because Asser not only mentions it, but describes the nature of the fortification. It was a _vallum_ drawn between the rivers Thames and Kennet, so as to enclose a peninsula.[134] It had several entrances, as the Danes "rushed out from all the gates" on the Anglo-Saxon attack. Such a fort belongs to the simplest and easiest kind of defence, used at all times by a general who is in a hurry, and it has therefore no significance in determining the general type of Danish works. Besides these eleven places where _works_ are mentioned, there are thirteen places where the Danes are said to have taken up their winter quarters, and where we may be certain that they were protected by some kind of fortifications. These are Thanet, Sheppey, Thetford, York, London, Torkesey, Repton, Cambridge, Exeter, Chippenham, Cirencester, Fulham, and Mersey Island. Four places out of this list--York, London, Exeter, and Cirencester--were Roman _castra_, whose walls were still available for defence. Three--Thanet, Sheppey, and Mersey--were islands, and thus naturally defended, being much more insular than they are now.[135] Three--Thetford, Torkesey, and Cambridge--appear as burgi in Domesday, showing that they were fortified towns. It is highly probable that the Danes threw up the first fortifications of these boroughs. There are no remains of town banks at Torkesey; at Cambridge the outline of the town bank can be traced in places;[136] and at Thetford there was formerly an earthwork on the Suffolk side of the river, which appears to have formed three sides of a square, abutting on the river, and enclosing the most ancient part of the town.[137] Chippenham and Repton were ancient seats of the Anglo-Saxon kings, and may have had fortifications, but nothing remains now. Chippenham is a borough by prescription, therefore of ancient date. At Fulham, on the Thames, there is a quadrangular moat and bank round the Bishop of London's palace, which is sometimes supposed to be the camp made by the Danes in 879; but it may equally well be mediæval. There was formerly a harbour at Fulham.[138]