Instead of keeping to the old plan of alternate harryings on either side, and precarious tenure of lands from time to time, AEthelflaed began building regular fortresses or _burhs_ all along her north-eastern frontiers, using these afterwards as bases for fresh operations against the enemy.

The spade went hand in hand with the sword: the English were becoming engineers as well as fighters.

In the year of her husband's death, the Lady built _burhs_ at Sarrat and Bridgnorth.

The next year "she went with all the Mercians to Tamworth, and built the _burh_ there in early summer; and ere Lammas, that at Stafford." In the two succeeding years she set up other strongholds at Eddesbury, Warwick, Cherbury, Wardbury, and Runcorn.

By 917, she found herself strong enough to attack Derby, one of the chief cities in the Danish confederacy of the Five Burgs, which she captured after a hard siege.