A curtain wall is a defensive wall

Defensive wall

A defensive wall is a fortification used to protect a city or settlement from potential aggressors. In ancient to modern times, they were used to enclose settlements...

 between two bastion

Bastion

A bastion, or a bulwark, is a structure projecting outward from the main enclosure of a fortification, situated in both corners of a straight wall , facilitating active defence against assaulting troops...

s of a castle

Castle

A castle is a type of fortified structure built in Europe and the Middle East during the Middle Ages by European nobility. Scholars debate the scope of the word castle, but usually consider it to be the private fortified residence of a lord or noble...

 or fortress.

In earlier designs of castle the curtain walls were often built to a considerable height and were fronted by a ditch

Ditch (fortification)

A ditch in military engineering is an obstacle, designed to slow down or break up an attacking force, while a trench is intended to provide cover to the defenders...

 or moat to make assault difficult.

Later with the introduction of trace italienne fortifications the height of the curtain walls was reduced and beyond the ditch additional outwork

Outwork

An outwork is a minor defense, fortification, built or established outside the principal fortification limits, detached or semidetached. Outworks were developed in the 16th century, such as ravelins, lunettes , caponiers to shield bastions and fortification curtains from direct battery...

s such as ravelin

Ravelin

A ravelin is a triangular fortification or detached outwork, located in front of the innerworks of a fortress...

s and tenaille

Tenaille

Tenaille is an advanced defensive-work, in front of the main defences of a fortress which takes its name from resemblance, real or imaginary, to the lip of a pair of pincers...

s were added to protect the curtain walls from direct cannonading.