In the early 10thC Chester, a former Roman town on the river Dee in N.W.England, which they called Deva, was a vitally important burh -it was a prosperous Royal mint lying on the river Dee, a noted and major Mercian stronghold and was geographically crucial to England's defence- being sited near to the northernmost tip of Mercia and near to E.Wales/S.Cumbria/S.W.Northumbria.
It also was sited relatively close to the estuaries of the Wirral, from where the vikings had attacked by sea from Ireland/Scandinavia since their ousting from Dublin in 902.
It's possible that the Danes of York were asking for Aethelflaed's military help in the 910's against roving Hiberno-Norsemen- Ingismund had been granted land soon after 902 by Athelflaed, but he betrayed her trust in 905 by attacking Chester- according to tradition, the warriors there repulsed the Norsemen by pouring honey onto the wall-scaling foes and unleashing hives of irate bees upon them!
King Edward, and moreover his sister Aethelflaed, saw to it's massive refortification.
Since 907 the 'Lady of the Mercians' had refortified the burh and manned it with troops as a prelude to their brilliant military 'Reconquest' of England (910-918) mirroring the string of new and/or refortified military forts (maybe built around this time, but not by Edward?) which strategically studded the northern Mercian border with Northumbria and Cumbria- one being the 'lost fort' of Brunan burh. Michael Wood calls this dangerous and highly volatile region as a 'dark age Vietnam'.
The six documented burhs in the NW of England that Edward and Athelflaed built/strengthened- all surround the Wirral were at;-
Chester in 907;
Eddisbury in 914;
Runcorn on the R.Mersey in 915;
Thelwall and Manchester in 919;
Rhuddlan (NW Wales) 921.
There may have been other unrecorded refortified burhs from this time, at Warburton (Cheshire) and furthest north of all at Penwortham (SW of Preston) on the river Ribble.
The local inhabitants in the Wirral and at Chester had even revolted against the Wessex Kings, and Edward the Elder was busy suppressing one such revolt when he died at Farndon-on-Dee in 924.
Edward and Athelflaed's retaking of Mercia, E.Anglia and the 'Five Boroughs' (Lincoln, Stamford, Leicester, Nottingham and Derby) from the vikings during their Reconquest of England (910-18) was truly fantastic.
Ironically, it could well have so easily come undone had Edward had a weak successor (he didn't- his son was the might Athelstan, but his nephew Eadwig was)- the vikings still roamed Northumbria, Edward was facing a revolt at Chester when he died in 924, and there was an attempted coup against his son, Athelstan, by another son (Aelfweard) over the next year.