The city walls formed the largest construction in medieval Utrecht. The five-kilometre long defensive works had to be modified regularly, which was obviously a huge undertaking..

For example, the walls had to be lowered and reinforced to permit the installation of cannons with iron cannon balls in the 15th century.

This renewal effort largely coincided with Emperor Charles V’s take-over in 1528. As an outpost of the immense Habsburg Empire, Utrecht had to have strong defences.