fort
A place built for the purpose of defending it against an enemy.
In that respect Edward I's Welsh castles were kind of forts, but the concept is usually distinct from a castle (which is also a community where people live, whereas a fort is merely garrisoned). "Fort" really belongs to an era of regular troops and national government (first use 1557).
Good places for forts are occupied territory, (to keep the natives down), as per Fort William, Ft Laramie, Ft Worth, etc. (in N America the term extended by association to mere trading stations) Why Forts often have this inverted naming standard I don't know. Not all of them do. E.g. Brean Down Fort, where I was yesterday, and which started me thinking about this.
Another place you build forts is on the coast to defend against invasion. e.g. Fort Cumberland (Portsmouth). Brean Down Fort was one of "Palmerston's Follies", a number of such forts built in response to the supposed might of the French navy and Napoleon III's sabre-rattling. It was built 1864-71, though by 1870 this concern about France was naturally at an end. The fort was staffed by 50 troops until 1900 when Gunner Haines fired a bullet into the powder magazine, apparently a spectacular act of suicide. From 1907 it was run as a cafe, then rearmed in WWII and used for weapons research; now it is unoccupied, but a natural destination for a walk. In January, the whole of Brean is virtually shut down and the idea of a cafe accessible only by two miles of hill-walking feels quite surreal. (But the one back at the road was open and I had a decent all-day breakfast unnecessarily beefed up with a side-order of chips - panic of impending starvation on what's virtually an island, especially with the road to Berrow closed.)
bastion - a sticking-out bit of a fortified wall, with the purpose of improving the field of fire e.g. to fire parallel to one's own walls, thus eliminating the safety-envelope of getting "Inside the range"
garrison
redoubt - enclosed defensive emplacement, often temporary and an earthwork, outside a larger fortification.
martello - a small circular tower used for coastal defence
presidio
rampart - an embankment, often topped by a parapet.
parapet - an earth or stone fortification to protect the torsoes of defenders. para-petto (It.)"for the chest".
terreplein - a level platform or earthwork behind a rampart or parapet, for mounting heavy guns.
Other words: redan, salient, re-entrant, pocket, annihilation battle.
(Adopts rapt second-sight-gazing posture and intones:)
A plant found on Brean Down in January so rare it doesn't even have an English name. Accidentally discovered by a botanist combing foreign debris out of a dried grass specimen, presumably Somerset grass. A little white lily or squill or something - otherwise known only from the Mediterranean region. I'm sure I haven't imagined reading about this somewhere, but I haven't been able to find any other reference to it. If you know anything about what I'm talking about, please get in touch!
Anyway no, I didn't see any.
[Not surprising. This was evidently a much-garbled memory of the discovery of Gagea bohemica at Stanner Rocks near Radnor - it is now known as Radnor Lily. Read about it here.]