NOTTINGHAM.--The first mention of a fortress in connection with Nottingham seems to suggest that it owed its origin to the Danes. In 868 the Danish host which had taken possession of York in the previous year "went into Mercia to Nottingham, and there took up their winter quarters. And Burgræd king of Mercia and his Witan begged of Ethelred, king of the West Saxons, and of Alfred his brother, that they would help them, that they might fight against the army. And then they went with the West Saxon force into Mercia as far as Nottingham, and there encountered the army which was in the fortress (geweorc), and besieged them there; but there was no great battle fought, and the Mercians made peace with the army."[109] Nottingham became another of the Danish Five Boroughs. The Danish host on this occasion came from York, no doubt in ships down the Ouse and up the Trent. The site would exactly suit them, as it occupied a very strong position on St Mary's Hill, a height equal to that on which the castle stands, defended on the south front by precipitous cliffs, below which ran the river Leen, and only a very short distance from the junction of the Leen with the Trent, the great waterway of middle England.[110] Portions of the ancient ditch were uncovered in 1890, and its outline appears to have been roughly rectangular, like the Danish camp at Shoebury. The ditch was about 20 feet wide. The area enclosed was about 39 acres. This borough was captured by Edward the Elder in 919, when after the death of his sister Ethelfleda he advanced into Danish Mercia, taking up the work which she had left unfinished.[111] The _Chronicle_ tells us that he repaired the borough (burh), and garrisoned it with both English and Danes. Two years later, he evidently felt the necessity of fortifying the Trent itself, for he built another borough on the south side of the river, and connected the two boroughs by a bridge, which must have included a causeway or a wooden stage across the marshes of the Leen. It is not surprising that the frequent floods of the Trent have carried away all trace of this second borough.[112] The important position of Nottingham was maintained in subsequent times, and it was still a borough at Domesday. THELWALL.--According to Camden, Thelwall explains by its name the kind of work which was set up here, a wall composed of the trunks of trees. This was another attempt to defend the course of the Mersey, which was once tidal as far as Thelwall. No remains of any fortifications can now be seen at Thelwall, which was not one of the boroughs which took root. But the Mersey has changed its course very much at this point, even before the making of the Ship Canal effected a more complete alteration.[113]