Fortress Many military installations are known as forts, although they are not always fortified. Larger forts may class as fortresses; smaller ones formerly often bore the name of fortalices. The word fortification can also refer to the practice of improving an area's defence with defensive works. City walls are fortifications but not necessarily called fortresses. The art of setting out a military camp or constructing a fortification traditionally classifies as castramentation, since the time of the Roman legions. The art/science of laying siege to a fortification and of destroying it has the popular name of siegecraft or 'siege warfare' and the formal name of poliorcetics. In some texts this latter term also applies to the art of building a fortification. Fortification is usually divided into two branches, namely permanent fortification and field fortification. Permanent fortifications are erected at leisure, with all the resources t...