fortified an unknown location called Bremesburh in 910 and in 912 she built defences at Bridgnorth to cover a crossing of the River Severn. In 913 she built forts at Tamworth to guard against the Danes in Leicester, and in Stafford to cover access from the Trent Valley. In 914 a Mercian army drawn from Gloucester and Hereford repelled a Viking invasion from Brittany, and the Iron Age Eddisbury hill fort was repaired to protect against invasion from Northumbria or Cheshire, while Warwick was fortified as further protection against the Leicester Danes. In 915 Chirbury was fortified to guard a route from Wales and Runcorn on the River Mersey. Defences were built before 914 at Hereford, and probably Shrewsbury and two other fortresses, at Scergeat and Weardbyrig, which have not been located. In 917 invasions by three Viking armies failed as Æthelflæd sent an army which captured Derby and the territory around it. The town was one of the Five Boroughs of the Danelaw, together with Leicester, Lincoln, Nottingham and Stamford. Derby was the first to fall to the English; she lost "four of her thegns who were dear to her" in the battle. Tim Clarkson, who describes Æthelflæd as "renowned as a competent war-leader", regards the victory at Derby as "her greatest triumph". At the end of the year, the East Anglian Danes submitted to Edward. In early 918, Æthelflæd gained possession of Leicester without opposition and most of the local Danish army submitted to her. A few months later, the leading men of Danish-ruled York offered to pledge their loyalty to Æthelflæd, probably to secure her support against Norse raiders from Ireland, but she died on 12 June 918, before she could take advantage of the offer. No similar offer is known to have been made to Edward. According to the Three Fragments, in 918 Æthelflæd led an army of Scots and Northumbrian English against forces led by the Norse Viking leader Ragnall at the Battle of Corbridge in Northumbria. Historians consider this unlikely, but she may have sent a contingent to the battle. Both sides claimed victory but Ragnall was able to establish himself as ruler of Northumbria. In the Three Fragments, Æthelflæd also formed a defensive alliance with the Scots and the Strathclyde British, a claim accepted by Clarkson. Little is known of Æthelflæd's relations with the Welsh. The only recorded event took place in 916,