The 'siege parallel' had been in development since the mid-16th century but Vauban brought the idea to practical fulfilment at Maastricht in 1673. Three parallel trenches were dug in front of the walls, the earth thus excavated being used to create embankments screening the attackers from defensive fire, while bringing them as close to the assault point as possible (see Diagram). Artillery was moved into the trenches, allowing them to target the base of the walls at close range, with the defenders unable to depress their own guns enough to counter this; once a breach had been made, it was then stormed. This approach was used in offensive operations well into the 20th century.