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 that a state can supply of constructive and mechanical skill, and are built of enduring materials. Field fortifications are extemporized by troops in the field, perhaps assisted by such local labour and tools as may be procurable and with materials that do not require much preparation, such as earth, brushwood and light timber, or sandbags.
There is also an intermediate branch known as semi-permanent fortification. This is employed when in the course of a campaign it becomes desirable to protect some locality with the best imitation of permanent defences that can be made in a short time, ample resources and skilled civilian labour being available.
Castles are fortifications which are regarded as being distinct from the generic fort or fortress in that it describes a residence of a monarch or noble and commands a specific defensive territory.