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 forts 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.