Moreover these walls were soon found inadequate to resist the missiles of the besiegers, and it became necessary to replace them by parapets of earth.

In order to cover the retaining walls of these parapets from the besieging batteries, it was also found to be necessary to lower these walls as much as possible, and to raise the counterscarps.

The traces or plans of the works, however, received no material change till about the close of the fifteenth century.
It is not known who first changed the ancient towers into bastions.

Some attribute it to an Italian, and with considerable show of reason, for a bastion was built at Turin as early as 1461.

Achmet Pacha, it is said, fortified Otranto in this way, in 1480, but whether the system was previously known among the Turks cannot be determined.