In preparation for the Bishops' Wars, further improvements were made to the defences: the ditch was cleaned out, and an additional ditch dug. Between the two ditches batteries were installed near the gates, and breastworks connected them to create a new outer perimeter. Drawbridges were installed at both Beverley Gate and North Gate, and the town was supplied with additional artillery. Sluice gates were also placed on the two rivers to enable the garrison to flood low-lying land surrounding the town. The historian Andrew Hopper described Hull as being "arguably the strongest fortress town in England."