Polygonal fort A polygonal fort is a fortification in the style that evolved around the middle of the nineteenth century , in response to the development of powerful explosive shells . The complex and sophisticated designs of star fort s that preceded them were highly effective against cannon assault , but proved much less effective against the more accurate fire of rifled guns and the destructive power of explosive shells . The polygonal style of fortification is also described as a " flankless fort " . Many were built during the government of Lord Palmerston , and so they are also often referred to as Palmerston forts . Their low profile makes them easy to overlook . In response to the vulnerabilities of star forts , military engineers evolved a much simpler but more robust style of fortification . The ditch became deep and vertical sided , cut directly into the native rock . It was laid out as a series of straight lines surrounding the fortified area that gives this style of fortification its name . The ditch was swept by fire from defensive blockhouse s ( caponier s ) set in the ditch , and firing positions cut into the outer face of the ditch itself ( counterscarp battery ) . The profile of the fort becomes very low indeed , surrounded outside the ditch by a gently sloping open area ( glacis ) , so as to provide no protection for an enemy while the fort itself provides a minimal target for enemy fire , and the narrow ditch a difficult target for plunging shellfire . The counterscarp , ( the outside of the ditch ) is usually vertical , while the upper edge of the scarp is steeply sloping and often revetted in stone , to help shed shells into the ditch . Access to the fort was down a curving ramp cut into the glacis , then through a gatehouse set deep in the scarp of the ditch , reached by a rolling bridge that withdrew into the gatehouse . The majority of the fort is underground , with deep passages giving access to the counterscarp batteries and the ditches defensive blockhouses ( caponniers ) from within the fort . Magazine s and machinery halls are deep under the surface , with only the emplacement s for the fort ' s gun s exposed at the surface . Perhaps surprisingly the guns were often mounted in open emplacements , known as " en barbette " , simply protected by a parapet . This was not only to lower the outline of the fortress , but also because guns of this period were rifled muzzle loader s ( RML ) , still using black powder as their propellant , which generated large amounts of fumes and smoke . Because of the fast burning propellant , gun barrel s were short , and accuracy still relatively low . Experience had shown that guns could be put out of action by collapsing their casemates around them by bombardment . The gun in its open emplacement was a much harder target to hit than the massive face of a casemate . The polygonal forts provided robust and defendable gun platforms . They were built in the context of a larger defensive scheme , as forward batteries to engage the enemy and prevent them bombarding more vulnerable targets , like city rail centre s and dockyards . Forts of this style were built extensively from the middle of the nineteenth century , but the end of the century saw the development of slower burning propellants , which allowed longer guns with much better range and accuracy and that produced much less smoke , and this along with the increasing reliability of breech - loading guns , favoured fortifications completely underground except for massive steel cupolas containing the guns . The increasing range of field artillery also required corresponding increases in the depth of zone that forts needed to be defend to prevent bombardment of the resources being defended . Meanwhile the development of machine guns and barbed wire offered a more flexible and much cheaper means of protecting from infantry assault compared to the polygonal fort ' s ditches and blockhouses . Many of the polygonal forts continued in service , even through the Second World War , since they provided convenient strongpoints , and their open emplacements readily accepted anti - aircraft batteries and quick firing guns , but the art of fortification had moved on .