sentences:  Inside the city, an earthwork was extended west from the end of the medieval wall at the inner north gate via Gloucester Cathedral to St Oswald's Priory, where it met the new outer earthwork.The outer north gate and Alvin Gate were fortified with earthwork bastions, a drawbridge was installed at Westgate Bridge and the Old Severn was dammed at Dockham, causing the ground to north-west of the city to flood.The garrison numbered no more than 1,500 troops.