Strategic value of fortifications

I’m watching the 2011 film Ironclad featuring Paul Giamatti as King John of England. The film is based upon a historic incident that took place in 1215 in which , the baron William d’Aubigny, Lord of Belvoir, led a small force that held Rochester Castle for seven weeks under siege by King John.

At one point, King John tells Tiberius, the captain of his Danish mercenaries*, that the castle controls the entirety of southern England.

At that point, I realized that I have no idea how castles work strategically. Thinking about it, I wondered why a castle would allow you to control large territories. The fortification protects you only if you’re inside the castle, so wouldn’t you just have control of whatever’s inside the castle walls?

Obviously, I’m wrong about that, otherwise no one would have bothered to build any castles. So, can anyone explain the situation to me?

*My other question is about these Danish mercenaries. I can’t find anything that tells me whether they actually were at the siege of Rochester.

Castles give you a protected place from which to sortie against…just about anything in range. Keeping an army in the field is a very expensive proposition. Without a base to return to, your forces rapidly lose effectiveness. With a castle in a key location, you have reasonably secure supply lines, a bolt hole to retreat to, and the ability to out-last your opponents.

Obviously John was exaggerating a bit about “controlling Southern England,” but it would definitely be a thorn in the side of whoever the opposition was. They would have to commit large numbers of men to protect anything they wanted to do in the area, and that gets pricey.

A lot depends on the era. Forts went in and out of style. Frederick the Great was brilliant at this kind of “positional warfare.” But Gustavus Adolphus brought mobile warfare to the pinnacle of the art. Forts were of relatively little importance in the Napoleonic era, but came back to prominence a while in the later 19th century. The Franco-Prussian war, and the Crimean War were “fort wars.”

WWI was positional… WWII was mobile… I’m betting that future wars will continue to be mobile…but anything might happen! Some jackknob will discover force-fields, and forts will get important again!

As **silenus **says, a castle provides you with a secure base to operate from. In terms of controlling an area, this can have a direct affect and an indirect impact.

A carefully sited castle can directly control a key point, in the case of Rochester this is the crossing of the River Medway on the direct route from Dover to London (vital for medieval English monarchs with their lands on both sides of the Channel). Indirectly a strongly held castle prevents the free use of a much wider area through the threat of sortie. For instance, without knowing anything specific about the Barons’ War, if King John had just ignored Rochester and gone off to bash one of his other unruly subjects, the garrison at Rochester could either have sortied to attack his army in the rear or sortied to harass those loyal to the King.

Strategically, building castles and other fortifications is a trade off. Generally they are expensive to build and almost never totally immune from attack. Even if physically very strong they can be taken by treachery or by starvation of the garrison but they do impose a constraint on your enemies. While your garrisoned castle exists it can’t be ignored - it either has to be taken, beseiged, or bypassed, all of which take time and resources which may be valuable.

Obviously the motive for building castles changed depending on the time and place. Following the Norman conquest scores of simple motte and bailey castles were build. Not enormously strong - basically a wooden palisade on top of a man made mound - they provided effective protection against a bunch of revolting peasants. A completely different proposition were Edward I’s castlesin north Wales. Again they were designed to deal with and hopefully deter rebellion but they had to cope with a possible major assault and were built to highest standard incorporating all the latest improvements in castle design.